"ava and the towship crew were really cruel to simon and it made him feel unsafe"
this might be explained by acknowledging the part of the movie where he personally admits to being responsible for the destruction of filament station. after the already-devastating mass extinction event of the quiet rapture.
nobody in any situation is going to be nice to the person responsible for that many people dying, and even if we take his assertion that it wasn't his fault at face value, the COI are still operating under the assumption that he was on filament as a hostile party that resulted in loss of resources (in an already resource-barren world), loss of life (after every habitable planet disappeared with the population still on them), and casualties (in a world that has limited space, technology, medical resources, food and air, and people with the skills and time to dedicate to taking care of others)
in the film, simon says "i want the deal" (implying that he was given a choice to participate in the exploration of AT-5 and took it, knowing what that entailed)
while ava and david are not explicitly nice and kind to him, (especially david, who leads with a friendly conversation that turns into him telling simon to go die, that he is replaceable, and to go fuck himself) they do provide him with the resources surplus to requirement for the expected duration of his dive.
they send him down with four bars of oxygen - he depletes only one in the expected dive time and is sent down again with the instruction to return 30 minutes later (supposedly one oxygen light's worth of time)
they provide him with drinking water (that he only needs to access after the dive time has elapsed several times over, explicitly after things have gone unexpectedly wrong)
he is provided with enough food for a meal, which again, considering the expected dive time and the COI's lack of expendable resources, is massively generous.
he is also provided with medical equipment including a pair of metal scissors (in a faction that is so depleted of resources like this that it's taken to salvaging from irradiated space stations and recycling their own equipment to build hemorovers), tape and bandages (and ethanol) far above the requirement for the expected injury risk of the dive, and a life jacket. this is not a cruel joke for the sake of humiliating simon and wasting resources for fun, it is both a sign of (at least captain ava's) intention to recover the convict alive in any state of emergency, and the absolutely ridiculous amount of hope that this entire expedition is running on.
simon is primed to believe that he is going to die for his transgressions against the COI. he does not seem to view this as a punishment and rehabilitation for a mass murder of human life - he simply believes that ava is fucking with him before killing him.
^ this is an eden belief.
we know that the eden station appears to have devolved into religious fanaticism around ritual suicide and murder. we also know that the COI, while acting a lot like a fascist military organisation, has constructed a conviction realisation program in order to integrate convicts back into (COI) society.
simon is treated, not like a prisoner waiting for execution, but like a disgraced member of ava's crew who is completing his sentence through doing dirty jobs (exploration of the blood ocean)
at no point does Ava, acting as Captain and spokesperson for the intentions of the COI, express any desire to see simon harmed. she expresses anger and resentment towards his complicity in murdering her people, and she jokes tastelessly with him about "something more calamitous" happening and "there just might be" something else down there, but that is a world away from sending him down to die.
within the events of the movie, simon indirectly causes at least three more people to be harmed, hospitalised, or killed.
he irradiates several members of the towship crew (despite being given the manual that explicitly documents the camera's function and danger, which, despite being down there for however long, he neglected to read). nobody is happy with him. they are still treating him with a level of sympathy and basic respect that he refuses to accept.
driving that hemorover around and exploring the blood ocean is simon's job. he is a religious cultist and a mass murderer, but he has been put into the conviction realisation program because the COI are desperate for more capable bodies. yes, he was given the job that nobody else wanted to do, and the people whose friends and coworkers he murdered were a bit mean to him, but he was given a choice and he took it, and he was provided with safety equipment and resources far beyond the margin of error that the circumstances required before the mission went unpredictably wrong.
simon is a sympathetic protagonist but he is not RELIABLE. he has no context and he exist within a framework of violence and cruelty, so of course he's expecting these new people to act the same way towards him. yes, david was mean and ava made some crass remarks, but he kept insisting that they were killing him even after it was explicitly stated that he was to complete his job and then be expected to continue doing his job whenever the situation required.
