I am not very familiar with Behaviour Tree’s yet but if I am hearing you right the following problem is happening:
The lawn mower moves toward the player and I am assuming knocks the player over and then stops.
And what you want is for it to run him over and continue moving?
If this is the case you might want to change the collision settings to overlap all dynamic. This will make the lawn mower trigger the “death” event when he comes in contact with the capsule you are referring to but will not stop the lawn mower from moving.
If you wanted the lawn mower to keep moving past the player to sort of “run the player down” you could then switch the move to destination to just past the character using some vector math if you had to, although I would guess that it might just do that automatically as is gradually slows down instead of coming to an instant full stop.
Try the collision settings first and let me know if that helps.