Obviously we're just speculating here... but given the name and theme of the character, Malice, the reason would likely be similar to the previous Mailce.
But there would have to be more to it than that. Like most good heel turns, there would likely have to be more to it than that (assuming they even go that direction of course... it's entirely possible NuNakia is dead and will never be seen again). An easy way to justify ooc behavior could be the resurrection alter changing her (and powering her up a bit). Or the symbiote if it ever finds itself off of Killmonger.