Why did Gus kill Victor?
There are two likely reasons:First, Victor was seen by witnesses at Gale Boetticher's crime scene. Gus being an extremely cautious man, thought it best to dispose of Victor to avoid him getting picked up by the police. A second reason which ties into the first; Gus killed Victor in front of Walt and Jesse to show that he was willing to kill a loyal member of his organization for a minor screw-up, what chance do Walt and Jesse have after their betrayal? Alternatively, In season 5, Walt theorizes that Gus killed Victor because he "Flew too close to the sun and got his throat slit.". Meaning that Victor started learning the process or cooking meth on his own by watching Walt and Jesse and in a sense, stealing the secret recipe. However, this was more Walt's wishful thinking as he was angry with Mike's attitude towards him. Lastly, it's possible that Gus suspected that Victor had been stealing the meth Jesse had been skimming from their shipments and giving to Badger and Skinny Pete.