Orcus on a bad hair day
Considering that this is what is done regularly with Conjure XX creations, including sentient creatures, and has never been consideed an evil act, I don't see this is any different.Well, of course that is your prerogative. However, as the spell forces the spirit to be loyal to the caster, it should do what it can despite any pain or discomfort it might feel either from direct damage or the forced ending of the spell in service to the summoner. The caster should understand the steed's role is a disposable creature, because its physical body is temporary, and the spirit can be summoned again later on, fully restored. Thus, only the form is disposable, but the bond/spirit can still be greatly valued--even more so due to the caster's understanding of the possible sacrifice/pain given in service. The spirit would understand this as well. It's (possible) pain in service to the caster likely prevents harm to the caster and is a badge of honor so to say.
Now, if the player throws their steed to the wolves (literally even LOL) repeatedly and shows no appreciation or remorse, they might not go insane, but should drift towards neutrality or even evil in alignment. Such acts, without feeling for the fate (albeit temporary) of the steed is hardly the act of a good person IMO. Of course, even then, they know their steed will return unharmed.
That is my take, for what it's worth.
If you want this for your world, that's great. And I'd play there - it sounds like an interesting twist. But since sending conjured beings to fight and die has been a long established part of play, there is nothing to stand on for general audiences that this now will shift the caster toward insanity or evil alignment.