Well, I think the problem with evil corperations is that they are frequently combined with a conspiricy-thriller mentality where the mystery and unknown questions are part of the appeal. They try and drag things along by acting like the guys doing this have some really good reason, that simply happens to be unfathomable at the level the protaganists are uncovering it, but dragging things out and refusing to explain anything, ever, without bringing up more questions prevents there being any kind of logic to the outside observer/player that allows for things to make sense. With the paticularly long running series, the guys writing them didn't even bother to try and justify anything anymore, because they went into it with the mentality that there was never going to be any answers, just the implication that some might exist.
To be entirely fair, I'm not entirely sure I can consider either Umbrella or Weyland-Yutani all that stupid overall, to give SOME credit where it's due, you have to understand that they both use some pretty sophisticated facilities and containment methods, which would not have logically failed. In the cases where such things DO fail it's usually the result of bad writing (painfully contrived, because if it wasn't, we'd never have a game/movie) or just never explained how the problems took place.
Two other points need to be understood about this genere as well.
#1: The protaganists are ironically the guys who screw everything up, something that makes no sense in cases where your dealing with characters that are supposed to be professionals and/or committed to doing the right thing. Half the problem with the Aliens actually seemed to be caused by the antics of the protaganists trying to stay alive and making things worse as they dragged the xenomorphs around everywhere with them. With Umbrella you'll notice that even with the first Resident Evil game things were reasonably well contained until a few STARS agents decided to be the ones to crack open the containment. In the movie it was even worse, the computer (Red Queen I believe) was trying to kill off the protaganists to prevent the infection from escaping. I believe they acknowleged that the computer was even right in like the third movie "kill a few, save a lot". From a certain perspective Umbrella had great safeguards, they had all kinds of lockdowns, and even an AI ready to cleanse the place if everything else went wrong. Arguably it was these professionals who kind of knew what they had locked in there with them figured "duur, let's get out of here, so we can live, assuming somehow nothing bad will happen if we break this containment". It's arguably Alice's fault that virtually the whole bloody planet got wiped out in the movie series.
One can talk about the human will to survive, but at the same time when your dealing with people who are already in professions where they know they are likely to die as part of their job to protect others, it becomes increasingly unlikely that some of these "plot nessicary" situations would arise.
It makes a little more sense when the people are spreading/releasing the infection are supposed to be "normal" but at the same time a lot of those people also witness what's going on. I honestly don't think your average person is going to callously try and break containment and kill a few million people or whatever, hoping that by some miracle the rules suddenly don't apply to them just because it's them.
In short, in a lot of these situations you can't nessicarly blame the "evil corperation". I mean Umbrella has been doing it's thing in underground facilities hidden under remote cabins on the extreme outskirts of small cities in nowhere land, or on private islands, or whatever. Weyland Yuteni is sending out teams of professionals usually (even if they are stupid in withholing crucial information, which is the dumbest part of the entire thing usually, since foreknowlege could change the entire series of events in some of these movies/stories), and all the action again
happens in pretty remote places.
Both Umbrella and Wayland-Yutani also do seem to try and clean up their messes, it's just that we are cheering for their enemies, oftentimes oblivious to the simple fact that as evil as the corperations might be on some levels, the guys we're playing on are probably more stupidly destructive. It's an odd way think, but consider those "evil corperate stormtroopers" that show up in some of this fiction were on standby to contain the situation, and they fail in many cases due to our "heroes".
#2: When it comes to motivations that's actually the easy part. Research and gaining knowlege for the sake of having it. Experimenting with the unknown is how you learn about it, and find useful things to do with it. I believe I remember reading somewhere (maybe it was a movie trailer) that Umbrella was hoping the T-virus and such could be modified for medical use to regenerate people and such.
I suppose if your a luddite, you can get into the whole left wing "Frakenstein" thing that demonizes any kind of radical research, that might be in any way dangerous, or actually has a chance to change anything. I very much oppose that and tend to be pro-technology, but I agree with a degree of responsibility, and on a basic level most evil corperations (and the two mentioned here) do seem to be fairly responsible in their set ups. In many cases I can't fault their security and preparations as being insufficient.
Now to make these corperations really evil, they do experiments on kidnap victims and such too, which I don't agree with, but that's usually not something that plays a huge role in why things go wrong. Usually your fighting a zombie outbreak or something, not from preventing a company setting out to be the second coming of Mengele on those principles.
When it comes to the motivation in "Aliens" I think half the problem was that a deep explanation went beyond the scope of the films, being someting that the protaganists you were following would never really get, as the series went on glossing over the exposition or only giving partial explanations which contridicted each other really added up.
With Umbrella you could pretty much define what they are doing as research for the sake of research, and trying to find ways to apply T-cells to other areas (bio-tech, military, etc...) that they already worked on.
With Weyland Yutani, it's actually been defined a bit better over time, but you REALLY have to watch a lot of these movies to "get it". The way it seems to break down nowadays given the prequels is that while something of a bastard Weyland happened to run into extraterrestrials on earth treating humans like fodder via the "Aliens Vs. Predator" franchise. While it wasn't spelled out perfectly it seemed like the Weyland running the show had some idea what was going to be found during the space mission that was the subject of the most recent "Prometheus" prequel, and if you try and tie it all together despite the differant writers and directors and contridictory visions, you could argue his corperation winds up realizing that the "Aliens" are basically highly evolved, self-replicating, biological weapons which have been used by both the species from "Prometheus" which apparently created them, as well as the Predator species that likes to dump them on unsuspecting planets and fight them for fun (and they still have a chance against this other xenos race). From a certain perspective you can see why getting control of the Aliens, or at least learning enough to make your own version you can direct better, to use as a weapon in a universe humans seem to be outclassed by other species (well, somewhat, by the time of the Colonial Marines, the military seems pretty effective), or at least learn to counter their biological technologies.... or at least this is how I'd put their general motivations together (and yes, given that the whole "universe" is a contridictory mess this doesn't work perfectly, but it comes pretty close). Ultimatly Weyland Yutani's biggest act of stupidity would be in not informing it's special teams/couriers/Military tools WTF they were doing and why. Of course while stupid, I suppose it's stupid in a realistic way as there is a lot to be said both for and against highly compartmentalized "need to know" information, some of the events in "Aliens" movies showing the downside.
