In the first Iron Man movie, what was Obadiah Stane's motivation for having Tony Stark killed?
Stane already had control of the board. Plus, it seems like Tony didn't really have a good grasp on all the ins and outs of what the company was doing. Was it just blind ambition and a lust for power? Or was there a bigger reason why Stane wanted him dead?
In the event of Tony's death, Stane would almost certainly have gained complete control of Stark Industries. Recall, he was Tony's effective mentor. He was Tony's friend. Tony had no remaining relatives, and few or no close friends. It's entirely likely that he would have given his three closest friends (Pepper, Rhodie, and Stane) the majority of his belongings. Most likely, Stane would have gained the majority of Tony's controlling shares. Coupled with Stane's own shares, it would have given him effective ownership of the company, as well as control of the board.
Tony was a brilliant engineer, capable of creating wonderful weapons. His latest, Jericho, would have revolutionized the arms industry and landed Stark Industries a HUGELY lucrative contract. All the other weapon firms would have been left (further) in the dust. With that kind of money, Stane could have hired lots of brilliant engineers to replace Tony. None would have equalled him, not even working together, but they wouldn't have had to - Jericho would have been Tony's swan song, and the boost it gave the company would have solidified it's lead on the arms industry for years, if not decades.
In short, Stane set out to kill Tony not for the Iron Man technology, which didn't yet exist, but for all the wonderful toys that already did. Stane saw that the company was about to reach a new peak, and decided to be on top of the mountain when it happened.
Greed was the initial motivation. As Xantec reminds me in the comments, Stane was the one who had Tony kidnapped in the first place. The implications are that Stane was tired of Tony and wanted full control of the company, as evidenced when he gets the board to start pushing Stark out when Tony gets back from captivity and his constant reminding of Tony that he was the one that kept the company from falling apart after Howard Stark died.
This is definitely not helped by Tony Stark announcing that Stark Industries was moving away from the arms sales game. With Stark Industries making a significant portion of their money from weapons sales to the US military at the time (energy via the Arc Reactor was merely a publicity stunt to Stain), as evidenced by their stock plummeting after Stark's announcement, Stane did not appreciate Stark taking the company in the "wrong direction" only furthering/reconfirming his belief that Stark needed to be removed.
Not only that, but once Stane had his Iron Monger suit, he would have this huge military asset that could not be countered... Except by Iron Man, aka Tony Stark, so he wanted Tony removed from the equation so that Stane could be the one who "privatized peace" to quote Stark from Iron Man 2.
So it was a combination of power-hungriness and greed.
Stane did not get control of the board until he had the emergency meeting to declare him unfit.
"Who do you think locked you out?" --Stane to Tony on the red carpet after Tony confronts him about the missle shipments.
So his motivation was to take control of Stark Enterprises. When the attempt to kill him didn't work, he used Tony's irrational behavior as a weapon against him.
