Tupac went at BIG cuz he was rolling with Bad Boy, it's simple as that. BIG didn't have the status in that clique to boss up on anyone and demand amends for Pac and wasn't interested in endangering himself or his family trying to do so. Even just giving Tupac information like he wanted would endanger him cuz Pac was liable to run his mouth about well Biggie told me this and that.
By him making that choice Pac felt betrayed and then saw it as fair to use him as a pawn in the larger conflict.
You can definitely argue that he was wrong for that, but that's the thought process on both sides at the start of the whole thing.