As per the rules for RDM, you can only kill if the person is a threat. Simply aiming the gun cannot be classified as a threat, the person must be advancing towards and show CLEAR intention to harm.
E.g. If the player walked towards you with the PSG-1 out and fired a shot, THAT is threatening.
Aiming for 30seconds and not shooting a single shot is a not threatening and is therefore not a valid RP reason to kill someone.
I would know, i've been banned for it many times.