Granted, when creating a story, you do put these sorts of elements into place. You may decide that Warduke, Chaotic Fighter and all around badass (despite questionable armor choices) has a soft spot for kittens.
You might then turn someone he wanted to kill into a kitten, in hopes that he adopts them rather than murders them. It's a contrived scenario, but so is deciding that Warduke slays who he wants, when he wants, and nothing ever gives him pause.
Ultimately, it comes down to whether or not your audience buys into the story you're telling. If the facts are too contrived, if there are too many coincidences at play, then they might reject the narrative outright.
But by the same token, something needs to happen to move the plot forward. Bilbo has to just happen to find The One Ring, and somehow resist it's corrupting influence for decades so he can pass it on to Frodo.
We're not going to make Will saves every day to see if this actually happens! Rolling low is inevitable, but we must suspend our disbelief or there's no story at all!