Gabriel was a monster to them now.
When Felix brought up ending Gabriel's reign of controlling other people, he mentions his mother as well as himself as part of the list of people Gabriel was turning into what he wanted them to be, and no wonder. Aside from hiding the existence of the peacock miraculous from Felix, the very jewel that controlled Felix's entire existence in the world, Gabriel was deliberately trying to make sure Felix and Amelie remained as naïve as possible, unaware of his true nature and the secrets he hid from them.
Ever since Amelie's first introduction in "Felix" it was obvious she had a lot of love and care for Emelie's family, wanting to be there for them even in their darkest times, but Gabriel for one could not care any less for Amelie.
Amelie may be Emelie's only sister, but Gabriel viewed her and the rest of her family as more of a nuisance. He hated Colt, Felix's dad, to the point of not attending his funeral, hated Felix for everything he did that led up to their confrontation in "Strike Back", where Felix threatening to expose his secrets if he didn't give up the peacock miraculous. And while Amelie made past efforts to keep their families close by visiting, Gabriel for one disliked having her around.
(Back in "Felix", Amelie planned a surprise visit to the Agreste' mansion, hoping they could all be together on the anniversary of Emelie's disappearance, something Adrien greatly appreciated, but not Gabriel.)
Aside from her timely visits, it was Amelie's willingness to confront Gabriel’s lack of attention over important matters within their families that only ever wound up clashing with Gabriel's secrets, such as the subject of returning his and Emelie's wedding rings, the very items containing Adrien's amok, but were rings that were still very important to her too as they were originally part of the Graham De Vanily family, not the Agreste's.
In "Emotion", Amelie did eventually questioned why Gabriel had become so cruel after Emelie disappeared, but she never imagined he actually cared so little about her and Felix.
When Felix set out his plan in "Emotion" to create a world free of anyone like Gabriel who would control others, it's shown at the end of the episode that he wanted his mom to open her eyes and realize that the people she thought were her family and friends, were all just monsters.
Back in "Multiplication", when it was shown that Amelie was secretly hiding Felix at home after the events of "Strike back", Amelie never told anyone that Felix was never missing, and yet despite the heroes questioning and regardless of his actions in "Emotion", Amelie never saw Felix as the bad guy, partly because she was his mom, but also because she had a great deal of trust in knowing that whatever her son was doing, he had his reasons for doing it.
Of course, Felix's past actions would not be easily forgiven by the heroes, he still had to face the consequences when the time came for him to return to the public eye, but Felix's willingness to reveal his identity as Argos to his mom, likely meant that Amelie was willing to give him the benefit of the doubt and trust her son on the matters of questioning who she was committing her life to.
When Amelie showed up to the Diamonds Dance, Amelie pointed out that a search had taken place for Felix after he went "missing", but despite her hopes, Gabriel and the rest of his friends never reached out to her to offer a helping hand. It's unknown how much Felix told Amelie about what happened in "Risk" and "Strike back", but regardless, Amelie got the point of why her son wanted her to question Gabriel's lack of effort in helping her. Amelie made a lot of effort to show she still cared for Gabriel and Adrien, even after Emilie disappeared, but it was Gabriel's, and the rest of his club members, lack of compassion towards her struggles in "loosing" her son that finally made her realize she had wasted much of her life caring about members of a high society who never cared about her.
Felix's decision to show his mom the reality of her connections, and thereby ending them, might have been crushing for his mom, but odds are, the final straw Felix had in continuing to stand by and watch his mom waste her love on people who did not deserve it, was the secret hiding under the Agreste mansion, Emelie.
(The shock of finding Emelie's body under the Agreste mansion, was enough to nearly make Felix blow his cover when he was pretending to be Adrien in "Risk")
In "Risk", Felix found Gabriel's secret underground lair, the very place Emelie's body was being contained, but at the time, Felix was only searching for the peacock miraculous, he never imagined Gabriel was secretly hiding Emelie's body under his mansion, and this was of course extremely horrifying to discover, because he and his mom thought Emelie just straight up disappeared one day, that no one, not even Gabriel, knew what happened to her. Adrien, Gabriel and Nathalie went through their own grief after Emelie was no longer able to be part of their lives, but for Amelie, that grief would have been just as terrible to bare when she found out she had lost her only sister. Much like Adrien, Amelie had to live with knowing that she had to accept that Emelie was just gone, she would never know what happened to her, and Felix was there by his mom's side when it all happened. It doesn't seem like Felix told his mom the truth about Emelie, but given the severity of it all, he didn't want her to suffer anymore grief over the truth about his aunt.
Felix holds a great deal of respect for his aunt Emelie and obviously would have had his share of grief when he thought she was gone forever, but to know that Gabriel hid the truth behind Emelie's disappearance from his mom, along with everyone else, never giving her the chance she needed to know what really happened to her only sister and say goodbye before she finally succumbed to her illness, must have infuriated Felix.
But to know that Gabriel would even go as far as making threats to get rid of him and leave his mom to suffer the loss of another loved one ("Gabriel Agreste"), all to obtain the ring that controls Adrien, and dissuade him from further pursuing his secret as Shadow Moth, only further convinced Felix that Gabriel was the one who had to disappear, before he made things any worse for his mom, himself and Adrien.
(In "Gabriel Agreste", Gabriel spoke through his sentiGabriel to threaten Felix. In this moment, Gabriel was no longer an ordinary uncle to Felix, he was a monster)
