Alignments are useful both for gameplay and setting. DND setting by default have extraplanar forces trying to influence the material plane along those 2 axes. Dealing with demons or devils, or even casting evil spells, contributes to the influence of evil. A good character who does such things is supporting evil, and will be influenced to turn evil themselves, even if they're doing it for good reasons. Vice versa applies also, evil characters can be tempted to the side of good by practicing good acts (even reluctantly, like as part of a ruse).
So we have in-game support for some classic tropes:
The lawman who keeps breaking protocol, becomes a loose cannon vigilante
The hero who makes sacrifices for the greater good, then for themselves
The rogue who "befriends" some naive marks, but finds comfort in their idealism and trust
The villain who "falls in love" with a hero, then falls in love because "woah compassion feels awesome"
Grey morality is fine, but it shouldn't let a character wield Book of Vile Darkness spells all day without becoming evil. The GM can allow it of course, but those spells are designed and described as tempting casters to evil. Casting them at all is an evil act, because (as a side effect) they increase the influence of evil outsiders over the material plane. And it's not just spells, of course, they just have explicit descriptors. Even if they don't seem to make sense, like Deathwatch being [evil].
So *I* wouldn't let a cleric of a good god use such spells freely. Or for a non-cleric, maybe one day their Luminous Armor (BoED spell, requires good alignment) no longer protects them.
A lawful good character could cast them too. Paladins are a class, not an alignment. The srd has paladins of all four extremes, even.
Also, reposting a private PM is an evil act
Taking a conflict to PMs is deescalation, and should be respected.