Ok, the question is simple, what would you do and how would you react in the following two situations?

First of all the setting is thousands of years in the future. Technology is much more advanced than it is now, things like robots (both giant piloted robots, and small automatic robots), mutants, laser guns, space ships and lightsabers are not an uncommon sight. That is all you really need to know.

Now, for the first situation imagine this. You are there minding your own business, doing whatever it is you always do (working, school, sitting on your computer) and suddenly ALIENS INVADE. These aliens aren't too smart, nor do they have a high level of technology, but they are fast and strong and can rip a person to pieces in seconds, that and they attack in swarms (100 to every person at least).
What would you do (assuming you don't get killed instantly and don't start crapping yourself)? Now lets say, you don't have access to a great deal of weaponry initially (you might scavange something off a soldiers corpse), maybe some sort of laser gun and/or a prized dueling lightsaber, other than that you have access to makeshift weapons, such as DIY tools, metal bars/poles and other objects (such as a table leg). Remember though, objects that might not necessarily be a "weapon" can stll be used to hurt things (setting things on fire, dropping heavy things out of windows/down stairs and so on). Also remember that this is a futuristic setting, you might have the skill to hack computers and gain access to things that can lock down areas, access security cameras or even take control of robots (if you are skilled enough the sky is the limit!). There also might be other people around that you can work with as well (this depends on where you are, but there probably is at least a few people around to help you).

Now, the second situation is a bit different. You somehow survived the alien invasion, and so did a group of other people (lets say 32 people including you). You wound up somewhere relatively safe, a spaceship in outerspace, or an underground facility (deep underground), it doesn't matter too much, the only important thing is that the aliens wont be getting to you anytime soon. However, one of the people in the group was infected by the aliens (might even be you who was infected) and before long will turn into a massive, vicious alien that will kill everyone else. But only the infected person knows who he is.
The infected person will try to survive, but so will everyone else. You can't go and just kill everyone because it will probably get you killed yourself, but you can't just let everyone else do as they please. You have to act suspicious without drawing suspicion to yourself, especially if you are infected. If you want to kill other people you either need a good reason (suspecting they were the infected), or make use of cunning by killing them when they are alone, or using some intricately designed trap.
In short, the infected has to kill everyone else without letting people know who he is, and everyone else has to find out who the infected person is and kill him without mindlessly slaughtering everyone else.
Remember, you have access to similar weapons, maybe even more you picked up along the way. The same sort of equipment (you could use security cameras to pick off your next victim, or to keep track of everyone else, you could maybe turn off the electricity in the area to disable all security systems and lights, making it easier to kill people and so on), the use of mind games would also be a massive help as you could trick people into killing innocent people to do your dirty work.
Also, the infected person has access to a few alien abilities, mainly increased strength and speed and better regenerative abilities. He might also be able to see in the dark, or have a layer of protective armor underneight his skin/around his vital organs or he might even be able to infect other people!

Anyway, in those two situations what would you do and how would you react?
And, those situations are not fixed. In the first one you could be anyone, from a person at school to a soldier in an army, you decide. But remember that depending on your situation you will have access to different things.
For the second situation the location you are will play a major factor on what you can do. In a secret underground research facility you could have access to all sorts of nasty traps, in an army base you might have access to all kinds of high tech and powerful weaponry (could be fights over who has access to said weaponry), and in a spaceship you might have to play it careful, you couldn't go around disabling systems and blowing stuff up because the spaceship could stop working, or even be destroyed in the process.