Why did Thorfin struggle more the second time?

So i found these two scenes to be quite similar, Thorfin has the opportunity to slash his father’s killers, but his self control takes charge and he spares them.

I don’t understand why the second time around, Thorfin seems to have more struggle controlling himself. I would think that because time has passed (like 10+ years) and he finally decided to not hurt anyone anymore, this rage wouldn’t consume him as much as it did back then. It feels like it has had the opposite effect on him though… Is there a reason for this? What am I missing? Thoughts?