I think the weakness he felt is only going to be a temporary thing. If they show his training Snoke will probably mention it and by the end of his training any guilt he will have felt will be gone. This isn’t the first time guilt has weakened someone who was of the Dark Side. Also due to his defeat by Rey (though circumstance lead to her victory) will feed his hatred and in turn will feed his yearning for the Dark Side.

He wants to be the badass in black armour like his grandpappy, but I don’t think he’s got it in him.

I mean he killed Han. That shows a willingness to fully commit to the Dark Side.

I think he’s just a scared, confused little boy like Rey said.

But who says he can’t evolve? That he can’t change like Luke did in the OT?