Rereading DN and this manga panel sequence is so funny yet a bit sad imo:
Matsuda after asking Soichiro if he shouldn't have said 'crimes have been decreasing' in front of the Kira task force and Soichiro replying it's no issue as uncomfortable facts need to be laid even more on the table.
Soichiro continues saying "Of course, it would have been a problem if you'd said 'Crimes have been decreasing thanks to Kira, so let's honor him for that'"
Matsuda: "I...would never say that, sir. Honor that psychopath...?"
And the panel that follows immediately is the aforementioned Kira doing his beloved sister's homework for her, spoiling her rotten coz he dotes on her and Sayu adores him and loves to spend time with him with the homework excuse.
I think it's a combination of sad and funny (in a dark way) I feel when I noticed this panel sequence. Kira who has killed so many criminals, identity being that of Light who has such a soft spot for his sister.
Of course, Light's actions as Kira are horrific as hell and I firmly believe they cannot and should not be justified...
He's still a kid though. A kid who had great expectations placed on him and he met each and every one of them with practiced ease. But his relationship with Sayu makes me ache, it's SO obvious he genuinely cares about her, even with him losing sleep due to his actions weighing on his conscience, having to play the part of a devoted top student, his entrance exams being not that far away- he still makes time for Sayu.
I don't think it's entirely due to him having a part of the perfect brother to play, but that he genuinely enjoys her company. He preens at her praise (she's one of the few people whose praise actually means something to him, otherwise he's just so used to being the best that he doesn't think much of it- his baby sister though? Now that's a different story), is comfortable enough to brag in front of her etc.
Sayu being too good for this rotten world is a thought I can easily imagine him having. Hearing her hatred for Kira breaks his heart...since he sort of agrees with her. He's not the ideal brother anymore, he still cares about her, still risked his plan to save her after she got kidnapped but he knows deep in his heart that if Sayu knew his identity as Kira she won't see him the same way ever again.
He knows his actions as Kira are evil. But Light so desperately wants to be a righteous person that he ironically becomes what he hates gradually.
Light being a multi-faceted character is so important to me. Him caring about his family doesn't make his actions as Kira any more justified, it does make me sad though.
And Matsuda not knowing that the 'psychopath' he was talking about, was doing his sister's homework for her at that moment is amusing to me.