Her reason is quite understandable actually
Alexstrasza in her eyes is a tyrant, enforcer of the Titans’s will, who destroyed the “old ways” and when they resisted, sealed them away in prisons, but they weren’t us lucky as the Dracthyr, they weren’t in stasis, they were well aware of the world around them; they were conscious. This is similar “mercy” what Illidan recieved from Malfurion. Kept in prison, conscious, without the ability to end your suffering and without hope to escape
And built her Dragon Empire - on the top of it herself as a self (or Titan Keeper) apointed Queen - and lived. As she said, time was exactly what Alexstrasza stole from her, from them and life. It is understandable, she want to see Alexstrasza suffer, want her to see how eveything crumbles around her, then maybe she will be allowed to die by the hands of Raszageth
Wrathion is another story; in here eyes, the “Black Prince” was basically nothing, a whelp, a brainwashed one, who have no idea what is to be a Dragon of old.
And the Incarnates and Primalist actually do have a point about the Titans, not to mention, her attack in the cinematics against Neltharion was a very calculated one; she went after especially his glove, the Titanic artifact he used to mind controll and keep the Dracthyr under his thumbs. You need a darn focused plan to aim your lighting to a human sized figure, souranded by Dracthyr, and especially aim that glove to shatter
I don’t think it was an accident she destroyed it; she wanted to free the dracthyr and succeeded and the Dratcthyr, arguably the greatest assets of the ragon Apects were benched.
In my humble oppinion, letting Wrathion alive was more than “I need someone to tell the world I’m back” cliché, since everyone and their mother saw her return. Simply she had no reason to kill some poor brainwashed kid, who was similar to the Drathyr in that sense, just without an artifact to controll.