in-game Joel is a traumatized, emotionally shut-down survivor who has succeeded as long as he has in a dangerous world because he's selfish. it's simple self-preservation. he avoids FEDRA mandated QZ duties, he and Tess are clear heavy hitters in Boston's underbelly, and he is disturbingly competent at killing people. to that effect, his violence is measured; decisive. practical. (this disconnection from his actions is arguably a result of trauma, yes, but unlike Show Joel, he isn't "triggered" to violence. he chooses it, because it's the most expedient way to stay alive or get what he needs. and in the tlou-verse, if you aren't the guy doing the violence, somebody else is gonna do the violence to you.)

this Joel has given up on his brother. they have not spoken in years, he has no idea if Tommy is still alive, and if he is, he assumes Tommy wants nothing to do with him. his primary goal is to keep his and Tess' existence as "free" and comfortable as possible, regardless of the nature of their relationship. (never made explicitly romantic, in-game.) he does not need to be "managed" by this Tess, like some kind of unpredictable guard dog, and the two of them together kill people handily, as needed, to maintain their place in the QZ pecking order. they seem committed to staying off FEDRA's radar, for obvious reasons. (FEDRA doesn't much care about protecting law-abiding citizens, let alone smugglers.)

so when Joel meets Ellie and is tasked with her transport, he is annoyed. it is a huge, hazardous inconvenience. & losing Tess is the exact kind of slap in the face he expected from getting involved in a battle he had no interest in fighting. he was not "already going out west" to look for his brother, and now he has to, in the hopes Tommy is still alive, and won't immediately turn him away because of their bad blood.

this inward facing selfishness is what we start to see chip away as Ellie--NOT "violence in her heart" Show Ellie, but traumatized, survivor's guilt riddled in-game Ellie, who is shy at first, and more jokey than mouthy--endears herself to him by being in parts clever, naive, hopeful, oddly sweet, and competent.

by the time they arrive at Tommy's, Joel wants to offload her not out of fear or self-doubt--convincing himself she'd be better off with Tommy and his guys is a lie to soothe his conscience--but because he's afraid of the bond they've formed, and is trying to run from it. he is being selfish to protect himself. (from more loss, pain, etc.)

this changes after the fight in the ranch house. he realizes he can run, but it's already too late, and actually they might as well finish what they started because yeah, he is capable, and maybe he's not her dad, but now? he's something.

and it's this selfishness, (love can be profoundly selfish) that motivates his heinous actions in the hospital. Joel, because of who he is, what he's been through, and how he gets attached, could not have made any other choice, world-be-doomed.

whether you believe killing Ellie would provide a cure or not (i do not, tbh, like, killing your one immune subject immediately is high stakes storytelling but terrible fucking science, w/e), the choice Joel makes is heinous and awful. and SELFISH. Joel. is selfish. (this is not me saying he is a bad person. the showrunners seem desperate to map our moral sensibilities to the setting, but that is ridiculous. Joel has to be selfish, or die.)

Show Joel... is self-doubting, and self-hating, full of regrets, (even suicidal????? WHAT??) and as far as we see, way less competent than his game counterpart. he's an emotional being. (that monologue to Tommy? i am not even going to touch it. he would not say that.) his attachment to Ellie seems almost instantaneous, and the showrunners treat it as an inevitable outcome, like these two characters are destined to meet and do violence together, which is, frankly, far less interesting than two very different people forming a close bond through incredible hardship that fundamentally alters the course of both their lives.