'American Idol' Alum Caleb Flynn Allegedly Cheated on Wife Before Alleged Murder

"American Idol" alum Caleb Flynn allegedly wasn't subtle about wanting his wife dead ... prosecutors say he texted his mistress about wanting to kill his wife and get away with it.

Caleb is standing trial on charges he murdered his wife ... and during opening statements Friday, prosecutors said investigators found more than 100,000 messages between Caleb and his mistress from the year before his wife Ashley Flynn was killed.

In one alleged message, prosecutors say Caleb wrote he stayed up until 4 AM "trying to think of ways to kill her and not go to jail" ... adding he'd kill Ashley "in a heartbeat" if he could do it without going to hell.

The night before Ashley was killed, Caleb allegedly texted his mistress, "I choose you. I'm free" ... then wrote, "Actions will come tomorrow, February 16, a day I'll never forget."

Prosecutors say Ashley was shot twice in the back of the head while lying in bed ... and accuse Caleb of staging the scene to look like a break-in.

His defense argues there's no direct or forensic evidence tying him to the killing, and says investigators zeroed in on Caleb after discovering the alleged affair. Caleb has pleaded not guilty.

As TMZ previously reported, Caleb called 911 claiming someone broke into their home and shot Ashley. Body cam later showed him distraught at the scene.

We also reported prosecutors expect the couple's young daughters, who were home that night, to testify.