Should the invader player be allowed to play only 1 card per "activation phase" or can he play all he desires for ultimate carnage?

By "activation phase" I´m refering to all cards saying "play at the start of your turn" or "play after a marine has declared an attack" etc.

Playing as many as he wants to in a specific moment of the game seems a bit unbalanced to me and the game might go from a survival battle to all-out slaughter just because the invader player has been saving up cards to really screw the marines over good and prober and just the right time, instead of spreading out their usage as he would have to if any single event only allowed 1 card to be played.

I don´t think it says anything specifically about this in the rules book but FFG implemented that line of thinking in android and to me that seems to work.

Any and all thoughts will be appreciated

Doom has no such limitation. Descent, however, added a rule that only one card could be played "per triggering event". For example, if the event was "When activating a figure..." then only one of those cards could be played. That's not a terrible house-rule to incorporate into your games (note that "Start of Turn" is not a triggering event, and the Invader/OL could play as many of those cards as he wanted to).

Also, are you playing with the original deck or the expansion deck. If you have it, I highly recommend the expansion deck, it tones the power level down a lot and makes for a better gaming experience...

-shnar