Kain vs Diablo

the best versions of each setting is hell. kain doesnt have his infinite source of magic

Problem here is youve not allowed other media, if you did I dont think Kain could win although tbh Diablo's actual feats are few, his speculated power is 1000x beyond planet busting.

How would soul steal work if Diablo didn't have the Soul Stone?

I dont know, the soul stone hmm. Probably wouldnt, Diablo would arguably be souless.