I don't think he was good exactly, but I think, in his mind, he thought he was doing something necessary. It doesn't make what he did okay, but we don't know why he did what he did. If it was power for power's sake, sure, that's pretty much my definition of evil. His motivation is important, and it hasn't been conclusively stated. Lea's statement is based on her knowledge and interpretation; she's likely right, but I still want to know why Justin was cultivating starborns specifically.
My take on it, fwiw.
Harry being starborn seems to largely involve, from what we know so far, power over Outsiders. The humans we have seen in the series display some knowledge and power connected to Outsiders are Cowl (summoning the Outsider-feeling pterodactyl/comet thing in the caverns at the end of WN), Justin (summoning the Walker to send after Harry), Lord Raith (running the Walker summoning, and whose protection from magic in BR Harry compares to feeling Outsidery iirc), and Maggie (whom Eb confirms in BR was a companion of both the previous two, and which is also supported by Harry being born in a "complex set of circumstances" to give him starborn power; that seems more than an accident of birth alone, and Maggie is the obvious person to set it up; Elaine's age being six months different from Harry's rules out it just being the date or else the DV would have hundreds of thousands of starborn).
So if we work on the basis that the latter three were allied to bring a starborn into being, and later parted ways over other disagreements, everything we see of Justin fits together. He finds Harry in the orphanage system, where Harry is by all accounts pretty miserable. He sweeps in to offer Harry the sort of sales pitch that is bound to be wonderful with a miserable small boy who so far as we know is already into SF and Fantasy (am I right in Harry mentioning having seen Star Wars when it first came out, which puts that a couple of years before Justin?). "Come live with me and I will be your father-figure and incidentally, you're really a special wizard and I will train you," almost immediately after Harry has his first experience of magic (in that long-jump competition) so Harry knows something odd is going on with him, but not what.
Then when we see the training bit with Justin, it's harsh, it comes with punishments and rewards, and it's pushing an agenda of "working hard at this makes you better than other people and you don't want to be a person of less worth."
And we know where that ends up, from Harry's flashback in I think it is SK, of human sacrifice, chalices of blood, and naked Elaine.
Following which we have Harry convincing himself Elaine was enthralled, going in to rescue her, and Justin trying to enthrall him. Note that Harry mentions in the confirmed-true memory in GS that Elaine only sat like that when she had a specific sarcastic point to make, so we have direct confirmation that there are other explanations than her being enthralled at that point. And note also that Elaine spends more than half of SK successfully part of a conspiracy to lead Harry around by the nose and direct his attention so that he ends up going to the Mothers and grabbing the Unravelling, so she has the skills to get him to believe things that are not true.
I reckon what's going on there is that Justin, whether as part of a larger conspiracy or not (I believe so myself but it makes no difference to this line of reasoning) is trying to raise starborn who will work along with his nefarious scheme, whatever that may be. I believe Elaine is in line with that scheme and Harry isn't, and there may be some collusion in there of "I am going to try and enthrall the guy but in case it doesn't work make sure he thinks you're innocent, he's in love with you so that should be easy" going on in that GS memory.
Then when Harry resists the enthrallment, the question of how much effort it is worth to try bringing him back on board needs answering. Sending the Walker after him is total overkill if he is just a half-trained apprentice, but also is pretty guaranteed to dispose of him. On the other hand, if he is not a loose end but actually is a starborn, that will show up fighting a Walker in ways it would not fighting a common-or-garden demon. Win-win, unless your Walker does his bit to spin Harry on going back and finishing Justin off. At which point Elaine fakes her disappearance in ways that convinces Harry she is dead for the next ten years; it seems very unlikely to me that he never once tried to use magic to find her in that span, so that has to be deliberate on her part.
And the end result of that is the Harry we see in the first few books. Extremely paranoid about authority figures, convinced that keeping information to himself is the best way of keeping people safe, placing his own judgement above anyone else's regardless of whether they might be better informed or not, and with temptation after temptation to cause mayhem thrown in his way (the Three-Eye potion, the shapeshifter belts, and ultimately, getting the White Council into a war.)
The direct, and fairly predictable if you know the people involved, consequences of Harry killing Justin help build his character too. The Council find him, they put him on trial, the Merlin wants him dead because that is what you do with warlocks unless you want to risk another Kemmler or worse (I am not sure what "worse" is but I see no reason to think the Merlin is lying when he says in PG Harry has no idea how bad warlocks can get, knowing full well Harry was up to his eyes in Kemmlerites in the previous book), and Eb intervenes to save him because he feels he has a second chance after messing up with Maggie, and succeeds because as Blackstaff he has stated-in-so-many-words licence to ignore the will of the Council. And if any of that does not seem a particularly reliable deal there is always Peabody in there to fine-tune how things go. Which leaves Harry as we see him in SF spending a lot less time than any other wizard his age around the Council, thinking they are a hidebound bunch of big meanies who want his head on personal grounds, and not growing up enough to really get a handle on Morgan's reasons for being so anti-warlocks until their confrontation in DB. I think the whole arc of the books, and why the continuing narrative starts in SF rather than with Harry as Justin's apprentice, is that we are seeing the slow painful uneven process of him growing to have more tolerance and understanding of people who are not like him, from a start point of having been deliberately pushed a long way towards sociopathy, arrogance, and elitism.
