He was a member of the Burial Agency. He is fairly duty bound to kill monsters such as Rider.
Well, yes, although it is hardly Rider's fault that she is like she is.
Also he would probably take care of the child that was being held by the monster afterwords, bringing her to live with the other kids.
Except that said kid would be trying to kill him, because, to him, Rider is not a "monster", Rider is
family. If you kill his aunt (or even attempt to do so), then as far as he is concerned you are a monster yourself, and nothing you say is going to convince him otherwise (at least if successful, if you fail it's possible that an apology and a promise to not do it again would be enough). Kiyoshi is only 12 or so, and he's pretty hot-blooded.
And, no, he can't be "disarmed" (because he uses magic), and nor is the fact that he's horribly outclassed going to stop him (because he's a kid, and his parents keep him the hell away from anything that can endanger him).
But overall there are far to many monsters in the Nexus City so he only kills those that threaten others. If he does not see Rider drink blood or kill he would probably let her go.
Ah, OK.
So if ya don't want conflict with the 4th member of the Burial Agency (ranked higher then Merem, who has 4 divine beasts) don't let him see Rider do those things, she probably picked up how to be careful especially around church people anyway through the years.
Well, she's careful about doing it full stop, doubly so when people are watching, and triply so when
Kiyoshi (or anyone else in her family) is potentially watching.
Killing Rider wouldn't be easy, though, she is still a Heroic Spirit.