He ruined Hector’s life and Atalanta’s life, and those are stories all of their own, but let me tell you a very summarized account of Why Penthesilea Deserves To Shove A Case Of Scorpions Up His Asshole:
Achilles and Penthesilea fought in the Trojan War. Penthesilea first fought Ajax, and then Achilles arrived. In one telling of the battle. Achilles pierced her breastplate and then stabbed her horse, making her fall to the ground. He then proceeded to mock her until he saw how incredibly beautiful she was under her helmet. Still, a battle was a battle, and Penthesilea was an immensely skilled warrior who had slain a massive amount of soldiers that day. Achilles stabbed her right in the face with his spear, with so much strength that her eyes popped out. The fact that her eyes popped out bloodily from the impact might be why she acquires red sclerae when she uses her Noble Phantasm:
Unhappy with what he had just done, Achilles, and this is where it gets fucked up, enamored with the slain Penthesilea, had sex with her corpse. Someone who saw this, a man named Thersites, laughed his ass off because what the fuck, dude, you are so messed up, fucking a corpse? Achilles did not take this mockery well and killed Thersites for it. Enraged, Diomedes, Thersites’ cousin, seized Penthesilea’s corpse and tied it to his chariot, dragging her corpse to the Scamander (a river) a la Hector.
Then, and only then, the Trojans retrieved her body and gave her a proper burial.
Long story short, she has a lot of reasons to be Very Pissed Off. This is not even counting her prior tragedies, including that time she accidentally killed her own sister. Since killing Hippolyta by accident, she had been stricken with grief, and only desired to die, hence her complete lack of self-sustain and suvivability skills in F/GO. As an Amazon, however, she could only die honorably in combat, hence why she was quick to help the Trojans in their war, and that’s when all the above happened.
All I am trying to say here is 1) she has every reason to be pissed at Achilles and 2) please be kind to her, she’s gone through ENOUGH.