I love the endless speculations here, lol. My take is that on facing Vader on the first Death Star, Obi Wan was not playing, he was trying to destroy Vader, and couldn't. He was briefly holding his own, but his age and lesser powers were no match for Vader. Upon seeing Luke, and having the prior knowledge from Yoda on how to preserve his spirit as a "Force Ghost", he knew it was his time, period. He knew that he would be come more powerful by being Luke's spirit guide and to help Luke on the path of becoming a Jedi and helping to revive the Jedi Order. Thus " You can't win, Darth. Strike me down and I shall become more powerful than you can possibly imagine".
I don't buy that Obi-Wan was trying to instill hatred in Luke towards Vader. He knew Luke was a young, impetuous spirit, who might try to go after Vader and would not be ready. Also, Obi Wan loved Anakin and felt partially responsible for his fall, and tried to distance himself from seeing Vader as Anakin, so he thought of him (Vader) as almost a seperate entity, "more machine now than man, twisted and evil."