Right, of course, yeah. It could do us all some good, save us a lot of fear and grief, save his future victims' lives - but this way, your conscience will be clear, and that's really the most important thing, isn't it.

[But he doesn't actually want to convince Watson to 100% kill the guy, so he amends:]

There are good reasons not to do it, but that - that isn't one of them, mate.