^
I hope someone brings up the fact Tara was on the opposing side of Rick's group at one point. Sure, she didn't open fire on Rick's group and knew she was on the wrong side once Hershel got beheaded AND she had to prove her worth to Glenn, but she could at least be more sympathetic towards Dwight, especially as he pretty much said he's aware they'll kill him after the fight's done. She should know how it is to be misled and to realise you're on the wrong side.
That said, they are completely different situations, and while Denise has been dead for like, 2 years for us, Tara only found out when she returned home in 7x06, which was day 602 of the apocalypse. Given day 610 was Mercy and only what, 2 days has passed since then? Tara's only been aware of Denise's death for 10 days, I can see why she's still pissed off and wants to kill him (well, more so given how fresh Denise's death is - especially as she's got the guilt of betraying Oceanside and the pressure to fight for her home).