My $0.02 worth:
For someone to be charged as a party to an offense committed by another, it has to be shown that he participated in some behavior that he knew was likely to result in the offense (in this case, murder) or was already engaged in another felony that resulted in a second felony.
I didn't read everything about that incident, but IIRC there was some bad behavior and words exchanged before the soon-to-be ex-marine shot the woman. Being rude and obnoxious are not elements of murder or another felony, and are not necessarily behaviors that automatically lead to a felony offense, i.e. shooting the woman. If they were unaware that he would shoot, or that there was a likelihood he would shoot, then they are not criminally responsible.
Now, if they were egging him on to threaten with a firearm, or telling him to take a shot, or knew that he was armed and encouraged him in some way that could foreseeably result in him shooting, that would be different, and it would have to be proved. Also, if they were participating in some other behavior that itself was a felony, and she died as a result, then I think they could be charged for her murder as well.
In something like a store robbery, unlawful force/lethal force are inherent to that offense, so everyone who participated in executing it would be responsible for the robbery, as well as any resulting deaths.
AFAIK this is the relevant part of the Texas law:
PENAL CODE
TITLE 2. GENERAL PRINCIPLES OF CRIMINAL RESPONSIBILITY
CHAPTER 7. CRIMINAL RESPONSIBILITY FOR CONDUCT OF ANOTHER
SUBCHAPTER A. COMPLICITY
Sec. 7.01. PARTIES TO OFFENSES.
(a) A person is criminally responsible as a party to an offense if the offense is committed by his own conduct, by the conduct of another for which he is criminally responsible, or by both.
(b) Each party to an offense may be charged with commission of the offense.
(c) All traditional distinctions between accomplices and principals are abolished by this section, and each party to an offense may be charged and convicted without alleging that he acted as a principal or accomplice.
Acts 1973, 63rd Leg., p. 883, ch. 399, Sec. 1, eff. Jan. 1, 1974. Amended by Acts 1993, 73rd Leg., ch. 900, Sec. 1.01, eff. Sept. 1, 1994.
Sec. 7.02. CRIMINAL RESPONSIBILITY FOR CONDUCT OF ANOTHER.
(a) A person is criminally responsible for an offense committed by the conduct of another if:
(1) acting with the kind of culpability required for the offense, he causes or aids an innocent or nonresponsible person to engage in conduct prohibited by the definition of the offense;
(2) acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense; or
(3) having a legal duty to prevent commission of the offense and acting with intent to promote or assist its commission, he fails to make a reasonable effort to prevent commission of the offense.
(b) If, in the attempt to carry out a conspiracy to commit one felony, another felony is committed by one of the conspirators, all conspirators are guilty of the felony actually committed, though having no intent to commit it, if the offense was committed in furtherance of the unlawful purpose and was one that should have been anticipated as a result of the carrying out of the conspiracy.