i swear i'm not saying this to be a contrarian or blindly committed to adore "evil women" but i don't think marial is as bad as people say she is.
she is between two worlds: serfdom and aristocracy. her actions are informed by the conflict in her morals and desires. having experienced the degradation of being a servant at court she has a better understanding of the cruel nature of the noble class and it was what made her realize that she had no true friends there.
yet she was born into nobility, and as such is accustomed to a certain lifestyle; to lose that lifestyle would be difficult for anyone, so if she was so close to having it back i can understand her panicking and playing the game of court politics by telling peter of the coup and hoping it works out for her. catherine was second on her priority, which she won't admit to herself but is very obvious to everyone else.
it doesn't surprise me that she kept the secret of catherine's mother murder because she doesn't like her. again, her principles aren't concrete but she's not worse than archie or orlo or even elizabeth. she's thinking of her convenience rather than how catherine would feel. but i also think the reason why she snaps and tells catherine about it is also because her father was stripped of his title because he... fucked peter's mother's corpse. he was punished for it and so was she merely because she's his nearest relation. for him to not suffer any consequences after all that he did, she felt the severity of its unfairness and it became less about wanting catherine to know and more about trying to get even once and for all. but this act is what made catherine realize that marial's loyalty is to herself first and catherine second. their friendship is second to marial's comfort, and so catherine gave her that: the comfort of living in the palace and remaining at court, without their friendship.
as a political move it's better for catherine not to keep her at such close proximity because marial's own gripes and grievances bleed so much into her opinions; she is not an objective person and cannot see the perspectives of others. the price of that move, however, cost catherine the very first friend she had, and it's clear from ep1 of s3 that she didn't enjoy setting that boundary between them, but it was a sacrifice that had to be done.
and yes, i do think that part of the hate towards her is because of the peter x catherine shippers. he's done far worse things than she has, he's just lucky to be portrayed by nicholas hoult. i'd be mad too if there were two chances to kill the idiot tyrannical emperor, whatever the consequences may be, and it just didn't happen and he continues to wander around the palace like he's on vacation.
her main fault is that she wants to have her cake and eat it too. she wants peter to be dethroned and dead but mostly for her own benefit. she supports catherine but doesn't consider that what catherine wants to do for russia will inevitably strip her of some luxuries. she supports the coup and reign of catherine so long as she isn't affected by it.
i'm still on ep2 of s3 (rip orlo, that was a very sudden and undignified death... i still haven't processed it) so i wonder how her arc will be handled here. i was a little disappointed by s2 on the whole so i'm not sure what to expect.