I maintain that the killer had the option of using his knife to a different part of the body, such as an arm or leg, which would’ve been enough to get away if he truly felt threatened. To stab someone not just in the chest, but in the heart, shows me he was trying to kill the other guy, not just get away from trouble.
This looks like PREMEDITATION to me; as though he had had it all planned out from before he even went to the meet!