I personally don't think Scar had the idea to kill Mufasa and Simba until after the "Scar tricks Simba" scene. I like to think that he spent most of his life dreaming of being king and simply hoping for Mufasa to die. But when Simba was born, he skipped the ceremony out of spite and became a bitter loner for a long time. And then, when Simba visits Scar and brags about being the next ruler, something snaps in him and he realizes he will never get to be king unless he acts. It is at
that point that I believe Scar decides to kill Mufasa, and Simba as well for good measure. Of course this is just my personal headcanon but I feel like it explains the sequence of events in the movie pretty well.
Even if we assume Scar had the desire to kill Mufasa from the start, there's no way he would have been able to do it himself, obviously, being physically weaker. I also don't think even the hyenas would have been any match for him. Mufasa was not only powerful, but wise enough not to get himself caught in any dangerous situations Scar would have planned. The only two times we see Mufasa being that careless was when he was saving Simba. Scar must have seen that Simba was Mufasa's weakness, which is why he basically used him as bait to lure Mufasa into the stampede. If Scar ever had the idea to kill Mufasa from before Simba's birth, I don't think there was ever an opportunity that allowed him to do so.
[quote="AnonLion"]I find it strange that only until Mufasa foils his plans of killing Simba does Scar decide to kill him too. But had he succeeded killing Simba would Scar still spare Mufasa? It feels almost as if Scar really didn't want to kill Mufasa but decided to do it just because he'd always be there to save Simba.[/quote]
I kind of disagree. I think Scar wanted to kill both Mufasa and Simba because they were both obstacles preventing him from becoming king. I feel like the Elephant Graveyard plan was meant to kill just Simba, leaving Mufasa to be killed by some other means. I don't think Scar foresaw Mufasa running in to save his son (and the hyenas were not expecting it either judging by their dialogue), but I do think it gave him the idea to kill two birds with one stone.