Help with Assassins creed syndicate on xbox series

In general, I go through Assassins creed syndicate on the box, there is a task in which you need to "knock out" the police. and in general I can not understand HOW to do it.
You approach hidden from behind and get the following picture:
X - kill
A - nothing happens
B - Capture
Y - shoots

If you capture with B then:
X- kill
Y - release shoot
B - release
A - hold to lead.

Damn, I can’t understand what’s wrong :( you can’t kill, I already zamael is not passable like that.