Being killed by a dragon isn't suicide in the relevant sense.
The AD&D PHB says (p 22) refers to "knowingly" performing a chaotic act and "knowingly and willingly" performing an evil act. The 3.5 SRD refers to "willingly" performing an evil act. Handing over the NPC is knowing and willing. Coerced acts are nevertheless willed acts (contrast automatism, or in the context of D&D magical compulsion).
The 5e SRD describes the Sacred Oath and Oath of Devotion as "commit[tin] the paladin to the cause of righteousness, an active path of fighting wickedness" which includes "protect[ing] those entrusted to your care". Giving up the NPC to the dragon clearly does not count as protecting someone entrusted to the paladin's care. (I am aware that the paladin in the OP is Oath of Ancients, not Oath of Devotion. But Oath of Devotion is clearly the most archetypal paladin.)
And for the sake of clarity: I am talking about what counts as doing the right thing and what counts as violating the paladin's oaths and obligations. I am not talking about the gameplay question of whether or not the paladin should lose his/her abilities (in the fiction) which is to say that the player should lose core elements of his/her PC (at the table). My understanding of 5e is that the notion of the "fallen paladin" following from a GM judgement call is not an inherent feature of the game. And personally it's something that I'm not a big fan of, as per the thread from 2011 that I linked to not that far upthread.
"Handing over the NPC is knowing and willing. Coerced acts are nevertheless willed acts (contrast automatism, or in the context of D&D magical compulsion)."
Wanting to understand this a bit, along with the comment about it not being suicide in this case to be killed by a dragon.
In this case, we have these facts in evidence from one side tedtimony.
Paladin was involved in world saving quest outside of this.
Paladin tried to carry some injured NPC to ssfety.
Paladin was caught and cornered by way beyond their power dragon.
Paladin tried to talk out of it, got "strong" result which led to the offer to give up the injured guy and leave.
Paladin agrees and lives - not a lot of detail about was he actually physically carrying etc during that part - the negotiation.
At this point, does it matter to you whether the paladin just walks away or physically hands over the injured? Is the paladin ok if he just walks away at this point? Was his fault the presumed physical act of "handing him over" or just the moral choice yo abandon him, stop protecting him?
Since the negotiation led to this offer, it seems obvious the remaining option either than the deal is fighting the dragon and both dead or just standing there waiting to be killed or accept the deal... barring of vourse some divine intervention.
Does the other wuest and its stakes and the consequences of falling here matter?
Finally, why is it evil acts are judged in the sense of immediacy, not longer?
Why isnt "instead of both falling here, let him go now, we lost this fight but not the war, then go work to rezz him and kill this dragon after saving the world..." a good act? Or at least a non-evil act?
This isnt end justifying means, this is seeing the difference in tactics and strategy... between a fight and the war - between a loss and a win.
I know you dont wsnt to engage on specifics of oath etc... preferring it seems more broad topic of evil... so ignore this last bit... but to me...
Seems to me that the whole ight of hope sorta thing is dependent not on dying when a fight goes bad, not suicide oneself into hopeless fight but picking oneself back up after a loss and turning things back around?