Jilted William Riley Gaul, 18, tweeted about his love for Emma Walker just hours before he was arrested on Tuesday.
His former high school sweetheart, 16, was found dead at her home in Knoxville, Tennessee, on Monday.
The young couple had recently split but Maryville College footballer Gaul allegedly refused to accept the breakup, the victim's aunt claims.
Jenny Weldon told the Knoxville News Sentinel: "She had chosen to move on.
"He refused to accept it. He chose not to accept her wishes."
Gaul is accused of shooting Emma through her bedroom window from outside her home as she slept on Monday morning.
He was arrested on Tuesday night after being under surveillance by Knox County Sheriff's Office detectives who feared her was preparing to destroy evidence, police said.
Hours before his arrest, the teenager posted at length on Twitter about his love for cheerleader Emma.
Gaul wrote: "To think that every memory we have, every happy special moment we shared, can't ever be relived. I love you Emma Jane Walker.
"The amount of pain your family, me, the community are in because of this is insurmountable.
"Every time we held hands, kissed, hugged, to imagine that I'll never have that ever again hasn't quite yet hit me yet.
"I'll never forget those times we talked about our future and what kind of dog we'd get and how awesome our family would be.
"I'll never forget sitting on the couch, watching Dr. Pimple Popper and making popcorn and laughing and holding you.
"You're beautiful, you're amazing, you're funny, compassionate, caring... you're my best friend.
"There's not a human on earth that can make the impact that you've had on my life. I miss you more than anything.
"Just knowing you have someone that's so in love with you gives you so much hope for your life ahead, and it can be taken away, just like that."
Staff and pupils at Central High School, where both Gaul and Emma attended, held a memorial service for the popular blonde student on Tuesday night.
While the vigil was held, Gaul tweeted: "I love you Emma, I can't be around any of that yet, it's too soon. I know you know I'm dying to be there but understand I can't. I love you."
Gaul is charged with first degree murder and is being held on a $750,000 bond in the Knox County Jail.