Take a look at how real world do it:
Indirectly trick other people into doing it themselves "for the right ideal". Except for extremely accurate divination, no amount of spells are going to catch the "true" criminal. And even on the slim chance you catch them that way, they can still do it in ways that you cannot prove it just by the way and methods they used.
Or just hire assassins and thugs. They tend to have no idea who hired them in the first place.
"Deniable Assets", I think those are called.
I mean, High-Fantasy politicians are probably all using that kind of methods already, since privacy isn't a thing in a world with divination spells.
And in a world where you have armed people roaming around murderhoboing their ways through life for money and fame, (a typical DnD-type character would be breaking how many modern laws just by existing again?) "whodunit?" is just a minor detail anyhow. v=w=v
That takes care of the methods and manpowers, the back-tracing aspects you can just use typical detective troupes.
(Think of it as kinda like clue-do: You have someone died. You have the killer. Now you need to figure out who hired the killer for what reasons and how you can bring in the real justice or prevent the next steps, of which can be further murders, some sort of coups, another step in a scheme, etc.)
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If its just your everyday "A bashed B's head and then try to cover it up"... That's just your usual combat scene that lasted 3 rounds.
