2.1 Random Death Match
Any player without valid reasoning cannot kill another player, appropriate reasoning to kill the player is needed. This is otherwise known as RDM (Random Death Match).
One example would be killing on suspicion, but you can find more examples of possible RDM at rules 2.5 and 2.6.
From your description of what happened you also did not see who planted the C4 so you assumed it was Chienman just because he came from a common C4 spot, by assuming and killing based on it you are killing on suspicion which rule 2.1 defines as RDM so the punishment is valid
Edit: In addition as Bezzy pointed out there is no evidence for or against your report and so it comes to your word against his and personally I am more inclined to trust a Head Admin who I know is very capable.