In what will be the first ever transgender hate crime prosecution in the United States, Vallum could be sentenced for life without parole when he appears before a federal judge in a Gulfport, Mississippi, courthouse.
Vallum, a member of the Latin Kings street gang, believed to be the largest Hispanic gang in the United States, secretly dated Williamson during the summer of 2014, according to prosecutors.
He decided to kill Williamson fearing that he could face retribution from other gang members if word spread she was a transgender woman, prosecutors said.
After luring his former lover to his father's home in Mississippi, in the United States' Deep South, Vallum shocked Williamson with a stun gun before stabbing her repeatedly with a pocket knife.
He struck deadly blows to Williamson's head with a hammer after she tried running away, prosecutors said.
This was the first case where a victim had been targeted because of gender identity that had been prosecuted under the US federal hate crime law, the Department of Justice said in a statement.
Vallum was previously sentenced to life in prison in a state court for the same murder.
But federal prosecutors brought an additional lawsuit for hate crime because Mississippi lacks a statute protecting people against hate crimes based on their gender identity, the Department of Justice said.
Harper Jean Tobin, spokeswoman for the National Center for Transgender Equality (NCTE), a Washington D.C. advocacy group, said the murder was part of an "epidemic of violence against transgender people" in the United States.