This is such an interesting moment.
Mello very genuinely shows remorse for what’s going to happen to Soichiro. He doesn’t really have a reason to lie besides creating a distraction, but I think what he was saying before this- “maybe I should have killed you when I had the chance”- was supposed to serve that purpose. Plus, Mello not wanting to kill Soichiro makes sense. After all, just because he’s willing to use any means to achieve his goals (remind you of L, at all?) doesn’t means he’s willing to murder anyone and everyone that threatens him- he’d find a better way to get them out of the picture.
That’s why this is one of the most powerful demonstrations of Mello’s resolve in my opinion, and it chills me to the bone every time. Up until this point, we’ve never seen Mello’s actually kill someone- he didn’t write in the Death Note and any deaths caused by him were indirect. Mello definitely is not the trigger happy, lit fuse killer that some people interpret him as, but this scene tells me without a doubt that if the situation had been reversed, Mello would have written Soichiro’s name in a heartbeat (pun unintended). I think Mello is sorry because he knew that Soichiro opposed Kira, and so in that sense, they were allies. He knew that Soichiro was just, and he also knew that was why he was going to die, because he wasn’t able to put aside right and wrong and do what is necessary.
That face in the last gif? That’s the face of someone who’s done what has been necessary.