SPOILERS AHEAD

Here is my theory:

Darkseid has been aware of Earth, secretly observing human affairs. He wants the planet but Superman is a detrimental factor in his plans. This can be for two reasons: his is either a great threat or a great ally. In reference to the latter, as expressed in Batman’s Knightmare, Superman loses his humanity presumably after Lois’ death, and ultimately serves Darkseid. In exchange, he gives rule of Earth to Superman. Though not without his supervision.

As a threat, Darkseid would have to kill Superman to make Earth vulnerable. When considering Luthor’s painting; the devils from the sky is an allusion to parademons invading Earth. As Superman is considered God, Darkseid would be the devil that Lex refers to. ‘If man won’t kill god, the devil will do it.’ So it is likely, Darkseid had some level of responsibility in Superman’s death. ‘ding ding ding, the god is dead’ possibly means Darkseid is now aware of Superman’s death and is now making plans for an invasion. Luthor sent the message before getting arrested.

After losing all options, he decided to contact Apokolips for help. Luthor was likely taught of Darkseid by the Kryptonian ship. It is probable that he came into communication with Apokolips off-screen, learning the most evil being in the universe has his eye on Earth. Luthor seems helpless in this case, being a pawn of Darkseid and creating a weapon he had absolutely no hope of controlling.

When he was speaking of devil's..I thought he was referring to doomsday

That's an interesting theory you got there.

Superman is resurrected as Darkseids servant and is the Villain of Justice League pt1....Bruce didn't have that dream/vision for no reason.....Brutaal cometh, calling it now