Murder is implausible. If for no other reason than it would a complicated evolution with no time to plan under the circumstances. Almost nobody commits cold, calculated murder on an impromptu basis as a matter of convenience.
Might be as simple as the guy went to the ramp to barf so as to not foul the cockpit, and then fell out.
Skilled investigators can parse a witnesses' statements and get a pretty good idea as to what's true and what's not. Doesn't mean they can prove it though.