In the final episode of season 1, Vice President Clark leaves the spaceship taking President Santiago around the Solar System. The spaceship explodes near Io killing everyone on board. Clark is then sworn in as Earth's new president.
In later seasons, we learn that Clark was a power hungry tyrant who ran EarthGov with an authoritarian and militaristic attitude. (In the very scene for the image below, we hear an off-screen voice congratulating William Morgan Clark on becoming president. I'm unsure if that voice was Ed Wasser's, the actor who played Mr. Morden on B-5.)
By this time, Mr. Morden was already contacting people in power around the galaxy asking "What do you want?" In the same episode where Santiago died, he and his associates even arranged the deaths of 10,000 Narns as a gift to Ambassador Londo Mollari, to make Mollari seem more powerful to other Centauri.
Was there any evidence Mr. Morden was behind the assassination of President Santiago? This is the sort of thing he would do. He would kill somebody just to help a power-hungry person with a desire for glory take over. He thought nothing of killing 10,000 people just to start a war.
Please provide answers based on canon sources. (e.g. - screenplays, official novels, interviews with writers, etc...)