Do you mean in a particular circumstance, or consistently over one's entire life?
It's complicated, because the nature of the "can't" here is problematic.
I don't think we're talking about ideals that a person can't conceivably live up to (eg we're not talking about cat trying to live the moral life of a human). Nor are we talking about the "can't" of causal impossibility - I think we're assuming free will, so that the character is free to choose and able to act in accordance with those choices. But equally I don't think we're talking about the "can't" of "well, actually, I guess I could have if I'd tried harder but I didn't, aw shucks".
The "can't" means something like "can't reasonably" - so when we say the ideals can't be realised in the real world, we mean something like "the ideals can't reasonably be realised in the real world". Note that "reasonably" here is an obviously normative notion, and so we don't have any evaluation of this claim that is itself value-free; the person who does evil on the grounds that goodness can't reasonably be achieved in the real world may just have a misguided conception of what is reasonable. (The selfish often do, particularly when weighing costs to themselves.)
Anyway, what is reasonable, given the nature of reality, depends very heavily on that nature. I think the providential outlook of a paladin or saint is important here: it shapes that person's understanding of what reality consists in, and hence of what is reasonable. Weber is one of the best authors on this, I think, though he is coming from the atheistic side. In "Politics as a Vocation" he distinguishes the "ethics of absolute ends" and the "ethics of responsibility", and argues that a good politician will follow the latter and not the former ethic, because it would be a dereliction of political duty to commit the polity on the basis of the sort of providential conviction that underpins an ethics of absolute ends. (The politician, says Weber, must aim at good in this world, not good in some other circumstance that lies on the other side of providential judgement.)
Now we can first think about Weber from the point of view of a paladin. The Weberian outlook seems an obvious instance of evil (or neutral? a difficult label to work with, for reasons already given). It is certainly not good, because it tolerates doing wrong because of a perceived need to choose between evils that disregards the balancing of the scales that providence will take care of.
But now let's think about Weber from the point of view of someone who agrees. That person does not judge the paladin - committed to, and following, the ethic of absolute ends - to be good. The paladin's not necessarily
evil, either - more like a fool or ingénue. (Weber in fact allows something like an aesthetic admiration for the paladin or the saint. But they are not to be permitted to be in charge of anything!) The ideals are fatuous, because the underpinning necessary to make them valid - namely, that providence will deliver - is fantastic, at best a naïve hope.
That's the sort of thing I had in mind when I said that "ideals that can't be achieved" entails "ideals that are fatuous". If the ideals can realistically be achieved, and are worth achieving, but are just hard, then it doesn't follow that they're fatuous. (This would be a common take among some practitioners of meditation aimed at the pursuit of genuine mindfulness - it's hard to do, much like running really fast is hard, but it is worthwhile in its payoff, and hence is not fatuous. But is also not something that can't realistically be aimed at, and perhaps achieved. It can be achieved, and it is reasonable to expect real people to aim at it.)
A further complication is that some ideals that are hard to achieve are ideals of the superogatory rather than the obligatory: the category of the superogatory would encompass many of the ideals of that religion you mentioned, and the reason for holding these ideals as superogatory and not obligatory may precisely be that living in the real world doesn't conduce to achieving them. (Some practitioners of meditation aimed at mindfulness might have
this view of genuine mindfulness - it's worth all of us aiming at it, but we acknowledge that many perfectly good people will fall short.)
But if the paladin who falls short of ideas because they're not realistic is simply failing in the domain of the superogatory, then s/he has not committed an evil act, or even a neutral act - s/he has simply failed to perform the best possible act - and hence s/he will not fall. (Which is not the scenario that was presented, and that motivated my original comment about fatuous ideals.)
Sorry that's a long answer, but the connection between ideals and realism is a complicated one. (Random factoid: the phrase adopted by John Rawls late in his life, to describe his political ideals, was "realistic utopia".)