I was going to do a 10 ton of painite hand off on demand for 5 million. Guy had a heavily modified engineered Python was refusing to refit it so he could only do 64 tons at a time. For that level of pay it was going to be about 300+ slaves. Which would have been about 6 round trips for slaves. I was on a time crunch and he was dilly dallying around. Here is the vid.
https://www.youtube.com/watch?v=11axg4fA0hE. I had 14 collectors which will pick up 60 tons of cargo in about 30 seconds from any angle so I was like drop the first load and he was all we need to be farther from the station and he kept leaving my field of view. So I'm in a stock Anaconda and he's in an Engineered Python and I'm flying without my rebuy and it just felt fishy. So I aborted the situation. I figured he might have a friend coming in on silent or he might be looking for a good angle to strafe or just wanted to frak around.
60 canisters is WAY too many to dump within the no-fire zone. Half of them would be deleted by the instance cleanup code the instant they're dumped. He was right to want to move 10km+ from the station, where the cargo limits are much less strict. Him moving out of your field of vision is also totally innocent - being positioned properly to jettison cargo toward the other person's cargo hatch speeds up cargo collection quite a bit. And frankly, if he was planning on ambushing and murdering you, he wouldn't NEED to lure you away from the station - there are much cleverer tricks he could have used to lull you into a false sense of security, like arranging the handoff outside a station that declares a no-fire zone but doesn't actually enforce it (yes, these exist, and are fairly common if you know what to look for).
From the information you've given, I'd say the other person's intentions were completely innocent, and that you were overly paranoid because you don't know that much about the game mechanics of cargo exchanging and how to set up an efficient exchange without getting screwed by the instance cleanup code. Of course, there was another contributing factor to your paranoia that needs to be addressed:
DON'T EVER FLY WITHOUT A REBUY. ESPECIALLY IN A SHIP THAT COSTS MORE THAN A HUNDRED MILLION CREDITS. Sell the Anaconda and buy it back when you can
actually afford it.