I don't really agree with this. Paladins of Tyr have been known to be zealous and a few have gone over the line, but they don't believe in innate evil unless speaking about Demons or Devils. Half-Orcs have become Paladins of Tyr in Forgotten Realms and Priests of Tyr have been known to deal with a redeemed Mindflayer Monk of Ilmater (had to copy paste that, sorry. And yes, I didn't like that bit either).
Tyr himself would slay a troll, but I don't think a Paladin should without consequence. If that Troll was not evil, no matter if it was a monster, it's still evil to kill on sight without chance of surrender. Tyr has been known to slay agents of evil, but that mostly is Demons and Devils. Prime world creatures usually have capacity to be good. Then again, I'm hardcore into Planescape and the ambiguity there makes Eberron look like a Saturday morning cartoon.
If a Paladin did that, when a monster is showing himself to be an ally and he were to slay the monster without speaking to the creature. I would hand out punishment.
I bolded the part that really stood out to me in this argument. "Without chance of surrender."
The combat had three sides. Orc, Troll, PCs.
The Troll, seeing some Orc at their weakest (Entangled and engaged elsewhere) jumps in to get an easy meal.
The Magic User sees the Troll enter combat, and knows that it is wise to handle the new threat quickly (trolls are usually a higher priority than orcs in my books).
He handles it as he would any Troll entering combat -> sets the bastard on fire.
Nothing Evil has happened so far on the party's side. Since the troll was knocked out during combat ((without surrendering to the PCs)), the bolded idea in your quote has no bearing. The PC's did not kill something that had surrendered, they killed a monster who jumped into combat with them.
Zealousness is Lawful, not Good or Evil.
The Half Orc's alignment is not "usually" or "always".
Mind Flayers are "usually" Lawful Evil.
Trolls are "usually" Chaotic Evil.
In my books, the "usually" phrase means that, while exceptions might exist, the average PC/NPC have never encountered it, and should assume that troll was under the effect of undetectable alignment.