As Maul is tagged as essential, he cannot be killed. As none of the law-abiding people of Riften care about him, no bounty will be added for attacking him, the guards will not intervene and may even egg you on. Great way to help levelling.
Also, If you have joined the Sotrmcloaks, or the Imperials and captured Riften for the Legion, the guards there will help you fight Maul. Since he is flagged as a member of the Thieve's Guild, Bryn, Sapphire, and any other member of the Guild will attack you. Unless you are apart of the Guild.
And for noobies, better have no follower(s) with you when 'training' on him... They'd ruin it anyway.