The problem with killing people who don't join you when there's other options is that eventually that gets out.
I can easily see the current White Council manipulating events to try and convince reluctant prospects to join. But they'd have to be very subtle about it. Because.....someone who IS capable finds out, and joins another group out of spite. Now what? They're protected, and they start spilling the beans about how the White Council manipulated them, killed their dog, threatened their family...you get the idea.
"Join us or Die." And then that nice guy with a spare coin goes, "Wow, that's pretty rude. Now, here's our offer. You don't have to take it; just hear me out, and if you want to walk away afterwards, no problem. WE don't kill people who say no."
"Join Us or Die." And then this Faerie Princess ambles by, contract in hand, which would guarantee your safety. (At least from the White Council...)
"Join Us or Die." And then...but you see my point. This could work for a while; but eventually, someone is going to get mad enough, stubborn enough, cantankerous enough, and last just long enough to join another group. And then they're going to talk...a lot. And maybe start offering their own rewards for joining them...
For that matter, this COULD be the origin of
The Black Council...maybe they weren't always the bad guys.
Perhaps the White Council has already created their nemesis...