A self-described methamphetamine addict who fatally struck a motorcyclist with his truck south of Half Moon Bay was found guilty of second-degree murder by a San Mateo County jury Thursday.

Mark Henderson, 49, was also convicted of willful disregard for human life, vehicular manslaughter while intoxicated, hit and run, possession of drugs and felony drunken driving. He could face up to life in prison when he is sentenced early next year.

“I think he earned his conviction and this sentence” said Deputy District Attorney Sean Gallagher, citing Henderson’s two prior arrests on suspicion of driving under the influence and his previous DUI education classes. “I think it was a long time coming.”

Prosecutors alleged that on Oct. 1, 2005, Henderson was high on drugs when he crossed into the opposing lane of U.S. Highway 101 near Tunitas Creek Road south of Half Moon Bay.

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Henderson’s vehicle struck 19-year-old motorcyclist Rebecca Siebenmorgan head-on. Seibenmorgan, a UC Santa Cruz student, was killed instantly.

Henderson’s attorneys argued that he was not high when he hit Siebenmorgan, but ingested the methamphetamine after the accident so police would not find it.

On Thursday, his lawyer Mitri Hanania said Henderson is extremely remorseful.

“He has been remorseful from the very start,” he said. The “first words out of his mouth when the sheriff’s deputy contacted after the accident were that he was sorry.”

tbarak@examiner.com