I am not the next child in line to say to parents things like, the importance of killing aliens as they might come to earth, or any other excuse. I do not agree with these either, but the surprisingly fine content comes from:
Coming back to the aliens, they rarely bleed, and there is an extremely low amount of blood on multiplayer, and is usually not possible to see unless looked for.
And the quality is excellent. Firstly, the story, while unlike one in a movie, is very high quality for a video game. The gameplay on single player, has very smart AI, and generally fun gameplay. Multiplayer is no less quality. And, I will not usually review games, put this one is exemplary.