Let's assume that Ventura was the best choice, ignoring his poll numbers, and Humphrey was way worse than Coleman. The defense for voting for Coleman is, recent polls showed that Ventura wasn't very viable. If Ventura then loses by one vote you can take comfort that you did the best you could. You shouldn't beat yourself up because your vote prevented a rare upset from happening. It was much more likely, given the info you had, that Coleman could beat Humphrey, so that you indirectly support Humphrey by voting for Ventura. (Keep in mind that a well conducted poll will typically have a margin of error of only 3% with high confidence. It's a "black swan event" to be off by as much as in this election. Probably the poll was flawed or voters had assessed info that postdated the poll.)

I don't like making these kinds of calls, though, which is why I support vote ranking or some other way to make third parties more viable.