I don't see that as the case here. Croc is evil. he only agreed to help Luffy because he knew he will get the best chance to take WB's head at mariford in the heat of the war. He knew he couldn't break out of Impel down alone either so he teamed up with the others. Note: If he could, he would have left them behind and went for WB on his own.
As for him saving luffy it was just coincidental like you rightly pointed out and like Daz said someone planning world dominion who will go out of his way to inconvenience an old man on a daily bases is just plain evil. Period.
And he did it just because he feels he can. He is a sadistic SOB who enjoys sitting on the sidelines and watching others he caused to suffer suffer. To me this is the worst form of how evil a human can get. The world nobles can't be excused for the things they do but they were brought up that way whether we like it or not but Croc is different he knows he is going to do something in his head he knows to be evil and he goes ahead to plan it and carry it out with relish.
I have to give him the respect he deserves here. He is the sovereign evil there is in OPverse.