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I can't respond to that without a torrent of profanity coming out of my keyboard.

Having listened to the referenced clip from the retired officer (I did not watch further into the video) I'd say that we have to leave room for the possibility that he does not mean anything like:

A: vaccination makes people into mass shooters.

It seems to me more likely that he is thinking (if we can apply that word to whatever lies behind his speech) something more like:

B: Those shooters were cowering at home in fear of the virus, so they had to suppress their homicidal urges. But now they are vaccinated they feel confident about coming out of hiding and committing the evil acts they have been longing to perform.

There is, perhaps, the background implication that mass shooters may be the kind of stupid cowardly liberal woke people who were taken in by the 'virus scam', and have been hiding away in terror while brave free thinking Americans were out there maskless and vaccineless. I would not be surprised at all if that was the case. But I don't think that amounts to him saying 'Don't get vaccinated, or you may end up shooting people'.

Was there more later in the video that contradicts this snap judgement on my part?

You're right, Chap. His argument was based almost entirely on option B, plus the background implication you suggested, plus, perhaps, the implication that the type of people who are unreasonable enough to resent having been pressured into getting vaccinated or did so only under protest because of having been required to by employers or other authority figures are more likely to act out their resentment by shooting people. But it is still a stupid argument against getting vaccinated.