Short answer: in the situation you've described, IMO it's probably best to let the fleeing perp flee.
You've stopped an
apparent felony assault and battery, but the alleged perp gets up and runs away. Well, at that point, it's hard to state the guy is a threat to you or the victim. At most, it can be claimed he's a potential threat to others. But particularly in a situation in which no actual physical harm (beyond a bit of roughing up) was witnessed, and no deadly weapon has been seen, then it's also tough to claim he'd be such a violent threat to the point that the only way to stop that threat would be to shoot. That's the sort of situation where forcible citizens' arrest situations get ugly, fast. Caution, if you want to go there.
For my money, given the risks when there isn't blood and/or a body as proof of manifest intent to kill, I find it hard to kill a fleeing person who's only crime was to have been witnessed assaulting another. If that person opts to flee at that moment, then I'm pretty sure I am going to let him do so. Daring the county DA and GJ to charge me doesn't seem like a good bet, in such circumstances, at least not until we get a whole lot further down the path to sane and rational support of citizens who are armed and act on behalf of others ... reliably so. Until then, if he chooses to flee, he gets a temporary pass ... at least until the 911 call brings police down around his ears in a few minutes; during which time, I might well tail him in my car (depending).
Now, if the perp was actually witnessed by me committing some egregious violence that couldn't be easily ignored (to the point there was a clear attempt to kill/rob/rape and either blood or a body), then everything changes quickly. That's when the choice of evils argument begins to hold much more water, in the case of deciding to put the guy down as he strives to rise (given the extreme risk he's manifesting intent to continue the fight/threat). Thin ice, in such situations, and you'd best be damned sure you're right. Else you could be spending some time in the clink wondering how you're going to pay for the charges being leveled against you, at least if you're in a county where you're in the least doubt about how it would play out.
Your call. All things considered, it's probably best in all but the most-violent and first-hand-witnessed cases to let a fleeing perp go.
Generally speaking, the
A.O.J. principles guide me. Specifically, the state statutes where I am do so as well. If there's no real Ability of the person to harm me, and no real Opportunity for him to do so (since he's fleeing, back to me, and apparently without a weapon), it gets hard to claim there's any actual Jeopardy to me at that moment. Hard, as in hugely expensive to me if I judge incorrectly. Those "reasonable man" elements in the statutes have teeth, all things considered, and here's a good situation where those teeth can be very sharp.
That being said, you're in Texas. Things tend to be a bit different there with respect to accepting the concept of stopping crime. Particularly if at night, you're going to have a whole truckload of support behind you, even though any shooting's going to go through the GJ. (ie, the case involving
Joe Horn, a couple years ago.)