•Well first of all, Itachi isn’t going to go easy on them given their history together. He just isn’t. If he has to kill them, or try to, he will. He’s in the Akatsuki now, he has his own personal mission, and whatever sentiment he once felt for them—possibly still feels for them—has to be ignored

•If his s/o is with a group of other Konoha shinobi, Itachi will try to bide his time with them first before trying to go after his s/o. If he has to counter his s/o, he’ll at least try to incapacitate them without killing them first

•But let’s assume his s/o goes straight for him in the heat of battle, out of some long suppressed resentment and a need for revenge because of the way he just abandoned them—it’s much more difficult for Itachi then. He can only evade them and deal out minimal damage for so long until he might have to cut them down. He can’t give the Akatsuki any reason believe he’s being lenient with Konoha ninja, especially not with someone who they might have previously known was his s/o. It’s just too risky. If he’s pushed to kill his s/o, he might have to

•If in the middle of battle, his s/o decides to give in to sentiment and start asking him things like “Why are you doing this, Itachi?” “Why did you leave?” he doesn’t have an answer. He doesn’t want to lie to them, pretend to be the villain more than he already has and come up with something that a true rogue ninja would say. So he just stays quiet. Talking to them would just make the whole thing more difficult

•So if Itachi isn’t the one who kills his s/o, then maybe another Akatsuki does, assuming Itachi isn’t the only Akatsuki in the battle. And watching someone else kill his s/o is definitely… rattling for Itachi

•He won’t show any emotion as he sees them fall. He doesn’t say anything. Doesn’t do anything. Just watches them die. Maybe he has flashbacks of their past together, when they were a couple in Konoha, before this entire mess. He’ll think about it for a long time afterward as well, but it’s just not enough to completely shake him. His form of grieving in this case is just to deal with it quietly and try to focus on anything else other than their death. At this point, he just has more important things to worry about. Hopefully his s/o can forgive him in the next life for what happened, that’s all he can really consider