The Watcher is the spirit of The Joker.
While some of his comments make it seem like he was never human, he also makes comments like the one about having always hated going to church. Plus, there's the voice and personality. Oh, and Wicked K mentions the Justice Lords. So, there's that. The "Mercy" gun War gets is a replica, thus being weaker
As revealed in the third game, Strife did work along side the makers. For whatever reason while it was made to help War, they couldn't make it to the same level of strength as the original. Death in the second game in contrast, finds one of the original guns. Vulgrim is Lucifer
He seems to have his own endgame, has access to powerful artifacts and the Serpent Hole network which he seems to be one of the few demons who can use and can allow access to others, he just wanted to look at the chosen hearts and made no offer on them, if he is Lucifer, he has enough personal power to not need them, or perhaps he only wanted a look to see how they work, He wanted War to kill the Destroyer because the Destoryer wasn't working out in regard to his plans, and punished Lilith because in DS 2 having interacted with Death, Vulgrim knew Lilith had failed. Samael will either replace Lucifer after the End War or kill him outright and become the real antagonist of the final game
It's a no brainer that Samael is planning something for his own gain. In Darksiders Genesis he refuses Lucifer's offer to power the Animus and corrupt mankind, so that he could manipulate War and Strife to slaughter his rivals and remain the only Demon Lord in charge. In the first game, he refuses to serve the Destroyer and this choice spares him from both War and the Council, the final result being the demise of Abaddon and the breaking of the Seventh Seal. Not to mention he also gives Death the Hell Key to open the Door leading to the Well of Souls, which results in Lucifer's plan to corrupt the whole Creation and using the Nephilim as his personal army falling apart. My bet is that he will eventually outsmart Lucifer and kill him early in the game or, in the better case, take his place as the new Lord of Hell after the Pact between the Three Kingdoms is forged anew.