This is semantics. Murder is a subset of homicide whose definition can vary by jurisdiction and country.
This happened in Texas so:
So now we have the definition of murder as defined by Texas law. Now, to be convicted of murder, under Texas laws, beyond a reasonable doubt is one way to define murder as a subset of homicide.
However, to state that just because someone has yet to be convicted of a crime under specific state law mean they are not is well.. a dangerous game. There is absolutely a difference between a convicted murderer and a murderer. However, just because someone isn't convicted, legally, of murder does not mean they aren't a murderer. On the opposite side, just because some IS convicted of murder doesn't mean they are a murderer. Hell, just last page I got slammed when someone assumed I believed all people convicted of a crime are guilty, which I don't because they aren't. Similarly, not all murderers are convicted.
Do YOU believe/think that there are people who aren't convicted of rape, child molestation or murder are rapists, child molesters or murders??
Would YOU say all people convicted of rape, child molestation or murder ARE rapists, child molesters or murderers?
I don't know your answer, but here is mine. I think there are a lot on both sides of that equation. Innocent people who aren't guilty and guilty people who aren't in prison.
Based on evidence I am aware of at this point, the offender has admitted to stabbing the victim. The victim died. That is, by definition of Texas law, murder, whether or not he has been convicted, if it can be proven beyond a reasonable doubt in court, otherwise. UNLESS, there are extenuating circumstances which I am not currently aware exist, and fully admit may well exist, then the offender is a yet to be convicted murder.
So, we can play semantics games, but using Texas law, based on everything I'm aware of to this point, the defendant appears to be a murder, just one who has yet to be convicted or plead. Will he be convicted, I don't know, but it seems likely that will happen or he will plead. Will evidence surface that changes my perception, I don't know, perhaps.
To cap it all off, in the US one is innocent until proven guilty, so legally, he is not a murderer, but he sure looks like one based on all the current information available.