Yes, Londo was never an "evil" character. The way I see him is he's just a regular dude. He's relatively intelligent, but his pride/culture gets in the way. Centauri culture is all about respect, status, nobility, and above all else, power. Londo is not in the big league power wise, but is high up enough to get some shitty ambassadorial post that the Centauri don't give a shit about (like becoming the US ambassador to Chad or some other bum nation). At first he's like, this is as good as it gets, might as well just party and enjoy it, but then he's offered a chance to showthe other Centaurithat he's not just a joke of a diplomat, that he can actually get some shit done and make a name for himself (every Centauri noble's dream) and makes a pact with the Shadows. Only he gets more than he bargied for, much more. That's why he confronts Morden when things get out of hand, but he's trapped now. That's why he starts to loath himself more than he did before when he was just a no-name diplomat.
He can't kill G'kar, even though he's a constant reminder of what he's done (the guy does have a conscience) - or perhaps even, he can't kill G'karbecausehe's a constant reminder. G'kar realises this (at least towards the end) and realises that fate has intertwined them together.
He was weak man in ever way and kept proving that fact. Not only that he was indecisive on what he wanted to be and wound up harming everyone around him and everything he valued most.
He's a good example of the fact that thoughts, feels and intentions mean little, actions and their results do and while he may not be an evil man valued just by his character, his actions were evil and devastating.
The sad thing is, he was someone full of potential and greatness but was blind to that fact yearning for something he already had inside of him. He could have been a huge villain and greatly benefited the Centauri or he could have shunned that and thrown his lot completely in with the rest of the cast but his incessant indecisiveness and constantly self-reenforcing perception of himself as someone being persecuted and unjustly overshadowed made him the worst of both.
He craved to be a great man to his people and make his nation strong, but lacked the fortitude to hold that course and see it through by supporting the Shadows entirely,he always strayed back into acting on his conscience but only when guilt was overwhelming all the while fretting, whining and demanding he be singled out and praised for his good actions and ashamed of the praise he got from his evil ones.
After years of trying to go the glory route, then doubling back and trying to be a good person doing noble things, he shows that he's not as much doing them simply because they're the right thing to do but that he should be lionized for doing than just as he got to see everyone else praised for the noble behaviour, only he constantly refused to walk the hard road to get that hard earned reward. You link shows that. He did a simple thing that was import, but what does he do? He goes to the person he's harmed the most in the show and bitches that he's not bowing and scraping to him.
A good comparison between him and pretty much the rest of the cast can be summed up in the episode where Sheridan and Delinn get screwed about by Sebastian and put to the test of sacrificing everything including ego, to potentially fail and note even become a mere footnote in history but forgotten. They we were willing to risk literally everything and they reap the rewards of walking that fine line.
How do you think Londo would fair being put to that test? No glory and recognition? What's the point in doing anything, even following your conscience if you don't get praised by everyone all the time for doing the right thing?
Pretty much the rest of the cast gets put through much the same test in different circumstances over the course of the series with G'Kar being hit with it the hardest and most if them pass (some fail in the end, like Linnear), but Londo never suffered personally (As in where it really mattered, somewhere about himself and what he values that is truly sacred - losing that chick he was fond for was petty revenge, typical crap Centauri have spats over), or at least not as much as they did, he never truly sacrificed choosing one part of himself over the other, instead desiring to breaking some eggs to make his great omelette, then panicking at the scale of the evil he unleashed and so bowed to his conscience. The end result is at the end of the series where he gets to see that his actions and swaying back and forth between the light and darkness earned him no praise, no recognition, universal scorn and a strong hint of distaste with those that still are his friend and respect him like Vir, his country has become a broken pariah as a result of his actions drawing the wrath of the Drakh.
In the end he's meant to show that, no matter what you do, you have to commit fully to it heart and soul. If you sacrifice everything for your principles and what is right then you become a Sheridan, if you sacrifice everything for the wrong reasons and seek to redeem, you do completely and become a G'Kar, if you seek to make yourself great and and powerful giving everyone else the big fuck you, you become an Emperor Cartasia, if you seek to bring your people back from the brink of decay and destruction, you become a Delenn, if you seek to simply be a good and decent person, you become a Vir, but whatever you do, you throw everything you are into that commitment and accept that you won't get everything you want in every way, but Londo refused to accept that he couldn't both become a great man sitting on a throne of skulls and be a paragon of virtue.
In the end, he was a fool and he knew it but he also knew that he could have been someone great and he had only himself to blame for failing to become that great man good and evil he might have been.