Not to tute my own horn here but the interesting thing is. Siobhan has been through this. She has been frankly a fighter since Adril recruited her for the Jedi after saving her life...her whole mentality was that of a soldier with force powers, which is why she did not fit into the Jedi Order and left and she's had to deal with the horrors of war, especially during the Bando Gora conflict.
She has always been a rather ruthless person when the situation calls for it. She tore down a roof over friend and foe alike at G99 during the Roche engagement because otherwise all OP/Republic troops would have died and to deny the Sith a tactical objective - hence why she calls herself the Butcher of G99. She's ruthlessly prosecuted the Bando Gora war with the objective of exterminating them and tossed a group of Bando Gora scientists into a cage full of Reavers, their own monstrous creations, as a form of summary justice, and ordering civilians to be gunned down because they carry explosive belts and have been brainwashed to attack her troops (they're innocent, but a threat). One thread that unfortunately never got finished involved her and Qae basically fighting an entire city that had been brainwashed by powerful sorcerers - so all the people attacking them are basically innocent, but cannot be reasoned with and so she had to butcher her way through to the 'daemons'.
She has touched the dark side several times...such as during her duel with a Primarch and also during her battle with Velok at Metalorn. Holding back is alien to her, she is - or rather was - Omega Pyre's battering ram and her job is to destroy the enemy, not to 'turn him or her back to the light' or angst about morals afterwards. She also authorised the use of torture for information during the Bando Gora war...carried out by the lovely Terminatrix Moira Skaldi.
Does she struggle with it? Certainly. She's a great fighter, but beyond that a very broken person and barely capable of functioning in civilian society because she has no clue about what to do with her life beyond fighting. She certainly does not regard herself as a 'good' person and she is quite dependent on the bottle by now. She certainly does feel regret for the deeds she has done...but that won't stop her from doing such again because war is not about who's right but who's left and honour is an excuse for stupidity.
This can be seen on Gehenna, the final battle against the Bando Gora, where the largest OP/ODF force engages the Bando Gora and wipes them out. Among other things using nerve gas to kill a large number of their Reavers and monsters. Of course, Bando Gora are monsters who eat people, conduct human sacrifices and inhumane experiments...but it's quite likely Siobhan would be just as brutal against conventional opponents if needed.
And in the last battle with Abaddon, the Bando Gora Prophet, the TK she unleashes against him results in the mountain ceiling coming down, again slaughtering friend and foe alike. Victory is achieved, but at a staggering cost (and Siobhan gets maimed...again). Siobhan will always go the extra mile needed to win...which includes sacrificing herself.
Is she 'lightside'? No, but then I've never been a fan of Manichean-style morality. She most assuredly is not a 'good' person, but then she's a soldier who has to make hard choices. @[member="Tegaea Alcori"] probably plays a big role in keeping her stable...without her Siobhan would be falling off the slippery slope and she's the person Siobhan cannot sacrifice and the one she absolutely trusts. As I have said often, Siobhan is destruction...on a leash. She would most definitely be an awful Jedi (well, the only reason she was a Jedi was for OOC reasons back on the old board I first played her at). Of course the temptation is always there, she who fights monsters and all that, though even totally Dark Siobhan would not join the Sith. But she'd go on a destructive rampage against them and anyone she thinks supports them.
But then like Tegaea I don't view the dark side as some demon ready to pounce on naughty Jedi if they misbehave - nor does IC Siobhan believe that. And 'redemption' for force-users doesn't cancel out their bad deeds.
Wow, this was long. Maybe I did put thought into this character after all!
TLDR: Sio is a mess and will do whatever it takes to win.