I do think he was more careful with evidence than we originally assumed, and perhaps thought he could get away with this. As perhaps he has before.
It's interesting though that he deposited the remains in bins that wouldn't be collected again for a week - that's risky. Most people would discover a bag in their curb bin and think - ok some jerk put his garbage in my bin last night - but would just leave it there until next week's pickup / not do anything about it. Because what are you gonna do?
It may have been total luck that Homeowner #1 actually looked in the bag and discovered the contents of the garbage were human remains. LE said somewhere that the remains were "packaged" and put into bags which may mean they were sealed in ziplocks in such a way that the smell would go undetected for awhile, and maybe even packaged as a disguise to look like something else. If this is the case, I'm definitely thinking this was well-premeditated.
Since he's not a homeowner, he might have assumed the bins were on the curb for emptying the next morning and so tried it.
I wonder why he put them in several different ones. Maybe because when he saw they were actually empty, he thought all of it in one would be too obvious and looked for another to split it up?