Question about the plot to The Commuter (2018) (family, spoiler, character)

[ironpony]

I just saw the movie and there is part of the plot I don't get.

SPOILER

Why did the villains get the main character, Michael, to try to kill the witness for them? Since Michael is an ex-cop, the villains had to threaten him with his family to get him to do it and he was constantly reluctant to do it.

But why didn't the villains just locate the witness on the train themselves instead of getting Michael to do it? Wouldn't that be a lot less hassle?