@ronri34
Tanzawa's smile could mean something like, "ha ha, oh I'm going to kill them all now." Or, he expected it to happen all along, and only gave Meguro an escort to cover his own ass so Kilihito doesn't blame him for driving Meguro away and neglecting him. If Meguro had died as part of Kilihito, it'd have been a severe blow to morale, but now it'll perhaps serve to motivate Kilihito's troops instead, since Meguro was beloved, despite quitting.
Another theory: Tanzawa loves feeling righteous when he fights (a bit like how Big Beanpole Bro gets off on feeling like he's protecting Lil Bro Nikaku) and loves going all out with the excuse "you hurt my friend," so when Meguru was killed (or when Tanzawa was told so, anyway) he smiled because now he has an excuse to feel good about going all out. For that matter, I don't think Tanzawa has forgiven the MC for fighting Kaname, I think he's just looking for a good excuse to fight him, one that won't make Kaname mad.