Law doesn't seek to punish but to rehabilitate. Act of taking freedom away from the criminal is violent enough. Treating them badly is just a sign of unfair/poor society that cannot maintain (afford to keep) it's promise to be civil to all citizens.
Because vengeance has never done anyone any good. You never feel better after getting vengeance, just hollow. Thus, a good legal system should strive to provide justice, not vengeance.
Have you ever distributed vengeance so you can personally speak how you felt? Or are you mindlessly repeating strings of words that are supposed to go together like an LLM?
I don't have to justify to you, random internet stranger. I have made my share of experience, and read a fair bit about that of others, in history and literature; and I'm confidently standing behind my opinion.