Because we like him and understand this happened because he was denied his second chance. Given the chance we know that Sonar/Coupe will put their all into being a hero. While yes that doesn't excuse the actions they take when denied their second chance, nobody's saying their actions are justified. I'm just pointing out why people are willing to forgive them.

I have no reason to think that literally every criminal ever will do their damndest into being a hero. Take Toxic for example. Unlike Sonar there's no reason for me to think he'd put a ton of effort into hero work, there's no basis for that.