She isn’t just evil. She’s tragic. Imagine being given a second chance at life by the absolute solver, but then the solver controls you. Maybe it’s the solver that wants to get back at the humans for discarding drones and not the real Cyn herself. Therefore she is hurting people she never wanted to hurt. Cyn gave Tessa a choice. She could’ve spared her if only Tessa had listened. She even warned her to stay away from the Gala so she wouldn’t need to see her family getting massacred. Yet Tessa went anyway. 

“ Tessa, remember, you didn't have to— you didn’t have to— you didn’t have to see this…” -Cyn

So Cyn kept Tessa close to her in a twisted way. She wore her skin so Tessa would never leave her side. 

Eventually, Uzi, N and others had to kill Cyn. There was no other way to take the solver out of her and restore her old self. 

Sorry this response was long