Well there is nothing wrong with camping.
If you get shot by a sniper a couple of times, while capturing the objective or on route to said objective, it should be clear to you that you need to find a way to take out that sniper before advancing. If you don't at least try, then you can expect to die a lot. It's basic logic we learn infants who mash the square block into the circle hole and cry because it doesn't work. Someday, the block or the shape might break and it will fit, but not without many, many attempts. If the game is objective based, it's even less clear to me why anyone would rage over campers.
Dealing with snipers in particular usually boils to down knowing the maps. If he got into the snipertower, that means you should have been watching the ladder that leads up there. If you keep getting shot on one particular spot, you now know that he has a perfect line of sight to that spot and he may not be able to see the other side of that piece of scenery.
Spawncamping imo, is only possible because devs allow it to happen. Most notoriously, you can get spawntrapped in TF2 in an almost inescapable way. If the defending team has an aggressive strategy, they can push you back into your spawning area so far they simply have to watch a single doorway. In the level called Dustbowl, on it's second and third stages, this will happen almost 1 out of every 10 times if the attacking team does not start out strong enough. This is simply because the defending team is allowed acces to the spawning area of the other team. In it's first stage, the level allows for three separate exits from the spawn, so the defenders have a harder time watching all three sides. Subsequent stages have separate exits to begin with, but they are not blocked off, so the defenders can phalanx their way in and lock you into your single-doored spawn.
Most of the times i get frustrated with campers is mostly due to bad level design. If the objective is only reachable through a single long hallway for example, then i almost immediately lose patience with the other team's sniper (if he's skilled enough to take on groups and avoid flashbangs and such). This is not his fault, due to the level design, he is automatically drawn to this spot because he wants and needs to defend his objective. Or if the sniper tower leaves no room to countersnipe him, because it's blocked by scenery. CodBlops had this immensely, because once a snipêr got up a tower, he can claymore the ladder and snipe away. This got frustrating on maps that did not have respawns, because you needed to sacrifice a teammate, just for the chance to take him out. This was inherent to the maps and gameplay, but what was also inherent in the game, you could also 'noobtube' your way past them because of splash damage. So i can't really stay frustrated for long.
On game modes that use a 'random' spawn location type thing, i will get frustrated by players who simply sit on one of these 'random' locations with a shotgun or the like. He's just waiting for RNGesus to smile on him and frown on me to spawn with my rectum already firmly placed over his barrel. This just means that once every so often, i'll die due to bad luck. Nothing to do with skills, tactics or team communication, but just frustrating luck. You can take him out once you know he's there, but i'm sure we've all met the player who simply runs back to his comfy little hole once he respawns.
All in all, i can't truly be mad at campers, since it's mostly either faulty level design or the other team's objective to find a way to take him out. I will say the occasional 'goddamit' or 'ofcourse', but that's just heat of the moment. I have never ever vented these towards other players though.