Congratulations, Adelina, this is a very interesting blog post! In fact, Lancelot's my second favorite character of all time, both in the show and in the legends(sometimes first there). He is usually portrayed as a tormented man who has to choose between keeping his honour and not betraying his king, or giving in to his love for Guinevere. Now, to Guinevere. In all the depictions, she goes with Lancelot on her own free will. Sometimes she is...how can I say it...a woman who just likes sleeping with men, sometimes she just marries Arthur out of a political agreement and loves Lancelot, and also leaves Camelot with him, and in some others she loves Arthur, but also loves Lancelot. Either way, Guinevere and Lancelot are some of the people that indirectly lead to Arthur's death.
What I don't understand is the reason you hate Lancelot from the legends. Is it that he came later? The legends change as time goes on, so new characters sometimes arrive and old ones leave. It's only natural. Or because you are an Arwen fan? Anyway, I don't think he is so perfect. As you said, in some depictions, he initially hates Guinevere because she steals Arthur from him(LOL). Jealousy and hatred are not characteristics of a perfect character. Also, he betrays Arthur for Guinevere and kills many good and bad guys alike. Just because his actions are justified it does not mean that he is to blame-if I find a way to justify Morgana and Uther's mistakes, does that make them perfect and not to blame?
Now, to the recent Lancelot. I have to say that I liked him in the Mists of Avalon(it may even be my favorite version)! Yes, he did ignore Morgana, because she was not as beautiful as Guinevere and because he had no feelings for her, but also they were cousins! Yes, back then maybe relationships between cousins were allowed, but it still creeps me out. Maybe it did to him too. Also, in that movie/books, Arthur is also depicted way too perfect (in order for Lancelot and Gwen to have their fun together, he had them have a threesome??? what the hell?) but I had no problem with his personality either.
I did not like Lancelot in 1981 "Excalibur", and kinda liked him in the Merlin movie. One of, if not my, favorite versions of him is in the show though. As Fimber said, he has the personality of a true knight! He, too, has done mistakes and is not flawless (like in the first episode when he illegally became a knight and fooled Arthur and the rest).
Finally, for the way the love triangle happened between him and Gwen...he was at times portrayed perfect, not wanting to ruin the relationship between Arthur and Gwen, but Gwen was also depicted perfect, not even thinking about betraying Arthur with a man she used to love. Between two perfect beings, the only way for something that happened in the legends to happen here as well, is magic!
Those are my thoughts on Lancelot in the legends and the show. Pretty long comment, but hey, he's one of my favorites ;)