For a franchise made by Disney and accused of being overly family friendly (mostly by fans of Zack Snyder's brand of DCEU darkness), there's been an awful lot of death in the MCU to date. Heroes, villains and innocents have all been slain in the interest of good story-telling, but there still seems to be an accepted logic that the real good guys would never stoop as low as to kill someone.
Surely Marvel's morally infallible heroes would prefer to incapacitate and imprison villains where necessary and would do anything to stop themselves directly killing innocent people, right? Well, not exactly. Pretty much all of the main Guardians Of The Galaxy and Avengers have at least one big kill to their names and only Black Widow, Hawkeye, Spider-Man and The Wasp seem not to (and even then, two of them are professional assassins, one helped kill Ebony Maw and the other is brand new to heroism). So there's a LOT of red in a lot of ledgers, basically.
Let's discount kills of Chitauri (since they're only partially alive anyway, judging by how they depower when their mothership is destroyed) and outriders (faceless mooks don't really count) and focus on the times the deaths REALLY counted...
Loki Killed Laufey & Coulson
Loki is a bit of a strange one in the MCU, but by the time he died in Infinity War, we were clearly supposed to consider him as a hero thanks to his last stand against Thanos. In that respect, he partly qualifies as one of the MCU's good guys, even though his affiliations have usually been defined mostly by what suits him best.
Even so, the reformed villain has two major kills to his name in Thor, in which he killed his biological father Laufey in defiance of his true heritage and then in The Avengers he killed Agent Phil Coulson and became the catalyst for the formation of the Avengers properly. Admittedly, that didn't stick, but he was definitely dead for a while there.