Worth buy Dead Island Riptide on -75%?

Yes

I bought franchise pack for 4 keys during fall sale from russian trader, so i say depends on the price that 75% off makes.

No, if you haven't played the first one. Play the first one first and for the record the first game was better than the second game. So i do no recommend buying the second one at all, just buy the first one.

I doubt it's going to go lower than 75% anytime soon. I don't think the base price will lower either, so it's probably the best deal. I don't know what it's like though. If you want to play it, this is probably the best deal you're going to get.

I can get this game for only for 2 keys.

No. My intuition suggests that it will have a greater discount or go into a bundle (again if you count that BTBTA) in the future.

The game is worth it in my opinion. Then again I am bias since I love both Dead Island as well as Dead Island Riptide.. so.. :P ( Oh and, I am one of those people who pre-ordered the first one and got Riptide during a sale. I would have pre-ordered Riptide as well if other things where not going on at the time. )

The first game is cheaper and in gameplay barely any different from the sequel. (Based on gameplay footage and friends)

Pretty sure gamefly had it for 4.99 2 weeks ago, wait for cheaper again.

I really liked the first one I played through the whole thing literally just myself. I heard that if you played the first one then don't get the second one because they're almost the same.