1. Although aller is used as an auxiliary marking a kind of future, it is also a verb whose meaning is to go. So here j’irai me venger means I’ll go and avenge myself and not I’ll avenge myself.

2. It’s just a way to put emphasis, somewhat hinting that others may not do that but that he, he’ll do it.

Note: j’irais is the form for the conditional and here we are in a context were the conditional makes little sense, it is most probably the futur j’irai (depending on the accent, both are pronounced the same, personally, the final vowels are [ɛ] and [e]).