Namor didn’t just kill Ramonda. Ramonda didn’t instantly die because of the water bombs he threw, she lived, and saved Riri, and then at some point in the moments between that and Shuri’s arrival she drowned, and Namor watched. He was there the whole time, just out of reach, hovering waiting for her death because he intended to follow through on his promise.
He didn’t kill her and then regret it because in his mind it was justified. Does it make him right? No. Yet Namor never once shows regret for killing Shuri’s mother. He watched her and Riri die and he waited while they tried to resuscitate them and only when it was confirmed Ramonda was dead did he leave. He doesn’t even care that Riri was alive at that point. He had done what he came to Wakanda to do, kill the Queen.
Honestly knowing this and seeing Shuri still extend mercy to him when he did not do the same to Ramonda just makes the final “show him who you are” moment from Ramonda hit so much harder, because it’s not about Namor in that point. Shuri could have killed him but she would also be killing the last piece of of humanity she had left.