This is a major misunderstanding leftists have of post-left politics. Most leftists are unable to grasp the difference between morality and ethics.
'Moral' is a label applied by people to themselves and their group so they can be perceived as a righteous being capable of doing no 'wrong'.
The 'moral' person sees themselves as fighting a universal battle between good and evil. They are the righteous crusader for good; incapable of straying from the 'moral code' that enshrines them in sanctified goodness.
The label 'immoral' is applied to whoever the 'moral' group decides is counter to their notions of goodness. They do this so they can maintain 'moral' superiority over the out-group and thus justify any action they take to marginalise these undesirables without feeling remorse or having to justify their behaviour to anyone.
The immoral villains can never be forgiven for their perceived crimes against morality because morality is definitive and final. The despicable villains must be forever shunned by the altruistic heroes in order to maintain their pious morals.
Racial segregation was considered morally righteous in the US South. As was cleansing the land of 'savages' during colonisation. Lynching bi-racial children for being 'impure'. Denying women equality by reasoning that it would lead to 'moral decadence'.
The recent government massacres of drug users in the Philippines were justified by creating a moral panic. The tyrant leading the massacres appointing himself as the one and only arbiter of virtue, that all moral people should blindly follow.
Perhaps the most deadly moral panic of the last century was spurred by Mao's cultural revolution in China. His Little Red Book of quotes; a virtual moral blueprint, was used by the party-faithful to purge scores of random people for having morally-objectionable... haircuts or fashion sense. Likewise, Stalin and his supporters in the USSR forced homosexuals into gulags where they were worked to death for 'crimes against morality'.
And of course the prototypical moral blueprint; the Christian Bible, was used to lead brutal moral crusades across the world; mass slaughters, land seizures and forced conversions of non-Christians.
Moral systems are designed to oppress and marginalise anyone the system deems undesirable. They are based on transcendent rules that are forcibly applied to all people from all backgrounds, in all situations; regardless of each individual's desires and values.
Unlike reactionary universal 'morals', ethics are decided on a case-by-case basis by the individual based on their own values and desires. Ethics are tangible and tied to real cause and effect outcomes.
A moralist opposition to violence is: violence is universally wrong, immoral, bad.
You might be daring enough to ask "Why?"
...Simply because the moralist says so. Requesting justification for such an abstract statement would be scoffed at because morality is seen by the moralist as some kind of divine truth that can't be questioned. The simple act of questioning it would be enough to render you immoral.
On the other hand, a measured ethical opposition to violence can be made by an amoralist... They can see that in many cases violence begets more violence, fosters systems based on the dominance of the strong, and can lead to deep-seated multi-generational divisions. But in other cases, they could see violence as ethically just. Because the alternative (e.g. fascism) would likely be worse.
A moralist forces their reactionary and irrational will on everyone else. An amoralist isn't concerned with forcing their personal perspective onto everyone, or with maintaining that perspective in every situation as if were unquestionable dogma.
Morality places paint-by-the-numbers judgement on every action, positing that all actions in column A are inherently 'wrong' and unacceptable, while all actions in column B are inherently 'right' and necessary. Regardless of the experiences of the people involved, their personal convictions and motivations, and the conditions that are present in that place and time.
Post-leftists aren't 'monsters' for rejecting morality. We're rejecting an incredibly flawed and reactionary concept that directly leads to untold misery.