First of all, Maytham was foretold by Imam Ali (a) how he would die and what his obligation would be.

Second, sometimes it’s important for the likes of Maytham to defend the Ahlulbayt (a), show the truth, and challenge the corrupt rulers of their time. So his circumstances were exceptional.

Third, he knew that Ibn Ziyaad would kill him anyway, so at least he wanted to take the opportunity to spread the truth.