Well, it depends on the specific comic series. In certain story arcs, Bucky is shown to have a role in that tragic event, but it's not a constant across all comics.
Comic storylines are fluid. Thanos and Tony Stark's interactions vary. So, there's no standard way Thanos kills Stark. It could involve cosmic powers, strategic battles, or unexpected plot twists.
The reason behind Captain America killing Tony Stark in the comics was a culmination of various factors such as ideological differences, personal vendettas, and external manipulations that pushed them to this extreme point.
It could also be a new villain who has somehow managed to hack into the Winter Soldier's programming and redirect his mission towards kidnapping Tony. This villain might be someone who wants to create chaos between the Avengers by pitting the Winter Soldier against Tony. They stay in the shadows, pulling the strings while the Winter Soldier and Tony are caught in the middle.