If the situation were that simple, yes. But in the real world, it never is.
For a start, you don't know why it would save "millions". You don't know which "millions". Basically, you need to be better informed on the scenario to make the decision.
For example, in the context of a time travel situation where you kill someone and it will save millions of people because said person never becomes able to carry out the act, you never know what other consequences there may be.
I know that some sociologists like to point out that if Hitler never existed, the diaspora from WWII would never have happened, and the current multicultural bent of Western society would be greatly diminished. That means that the Allied countries could all potentially have ended up as obsessed with racial purity as Nazi Germany, except within the context that they'd be the first to have nuclear weaponry.
It would be all too easy for nuclear war to occur in that box of fireworks. So you can see the kind of can of worms people can open in doing this sort of thing.
Fiction really explores this scenario a lot. What if you're being manipulated in being made to kill the guy? What if it's a trap to frame you for killing an innocent man? What if in saving millions, you also conversely kill billions from some unknown consequence?
The point is that if you're saving millions, that is a massive act and the consequences can be very far reaching, in fact, so much so that you can no longer assume that it's ultimately a good action. While saving life is absolutely an inherently good thing to do at the base, we never stop to think, "is one of those millions just as bad as this guy, or worse?" I'm not saying that it should stop you, since you probably wouldn't know at the time, but even that is a possibility.
You can ruin the future by thoughtlessly, instinctively or simplistically engaging in such an act. You would have to have a very sound philosophy behind your decision to even begin to have a chance of mitigating the worst possible results.
If you had no further information, and no other choices to gain more, and only just one chance, I suppose you'd be forced to do it by circumstance. Really, the person who informed you is the one behind the action anyway. In obeying, you would merely be a cog in the machine to them - if quite tainted by the process of murder.