Are you planning to take over his job afterwards? Cause regulating which dragons can stay in Waterdeep or not is quite a huge deal. These dragons may well aid or impede you if you try to kill Maaril depending on whether they believe you will be a better keeper of the dragonstaff of Ahghairon. And tbh despite their disparity in alignment, Maaril has been very courteous (and has probably saved Serenscales from Klauth without her knowing more than once). The Blackstaff is most certainly privy of this as well, and will not allow interference in the draconic status quo so easily; adventurous dupes who could be persuaded to let dragons in his city could end up with many dead waterdhavians.
In short, if you are planning to kill off Maaril, make sure you get the backing of the locals first. Or maybe offer an oath (or better a geas) to hand over the dragonstaff to someone appropriate.