Belkar didn't kill the Azure City guard in cold blood. He killed him (or was it her?) in the course of an escape from capture by an enemy. Perhaps not "good", but not "evil" either.

He arranged to have the Lawful Good Kobold killed because said Kobold was actively trying to kill him. Nothing evil there either.

He killed Grand Larceny Guy to prevent him from killing Hinjo. His actual motiviations were base, but his actions in that incident were not at all evil.

Poor Belkar. Just because he's CE, he gets blamed for EVERY little thing he does that no one would blink an eye at if another character had done it. If Roy had been in the non-anti-magic section, jumped out, and killed the guard, would anyone blame him for it? If Elan had been chased by Yok-Yok and maneuvered a group of adventurers into dealing with him, would we not be praising him for his uncharacteristic quick thinking? If Haley had shot Grand Larceny Guy, would we have objected? I don't think so.