To add to the above points you should really never run
Murder in a modern deck. The most common black removal spells in modertn are
Fatal Push and
Dismember. Mana efficency is key. For this reason you also sometimes see
Cut Down crop up, though I personally dislike it as it can't kill an online
Dragon's Rage Channeler.
Though
Fatal Push needs Fetch Lands like
Marsh Flats or similar to reliably enable Revolt. If these are not available for budget reasons, there are still plenty of two mana removal spells. At the cost of two life
Infernal Grasp kills anything. That's the one I'd recommend. Otherwise there is great amount of two mana removal that kills a lot but not everything like
Shriekmaw (two mana only if evoked, but extra value if you draw it late in the game),
Doom Blade,
Go for the Throat,
Cast Down,
Power Word Kill, ...
Lastly for three mana there is
Hero's Downfall which is just strictly superior to
Murder and since it was recently reprinted at uncommon really not much strain on the budget. Not strictly better than
Hero's Downfall but in my opinion still superior to it is the Swift End half of
Murderous Rider.
Though to
reiterate, mana efficancy is key, so I'd run at most two 3 Mana removal spells and the rest for less mana.