Basically what they did in the first one. You could kill reaper leviathans i think, it just wasn't worthwhile to spend that long stunning it, hacking away with the survival knife, and restunning it. Either way im glad killing the animals is off the table. Hiding, running, and working around the leviathans is how the main gameplay loop creates and releases tension. I wouldn't place subnautica in the horror genre, but the leviathan encounters are structured just like a horror game chase scene and that is an essential part of what makes the game exciting. If its possible to kill them then those tense chases get replaced with boring ammo collection in the shallows until you can have a boring torpedo fight with a whale that can't shoot back.