Because they're fucking evil.

According to the UN, Syrian armed and security forces may be responsible for: unlawful killing, including of children (mostly boys), medical personnel and hospital patients ("In some particularly grave instances, entire families were executed in their homes"); torture, including of children (mostly boys, sometimes to death) and hospital patients, and including sexual and psychological torture; arbitrary arrest "on a massive scale"; deployment of tanks and helicopter gunships in densely populated areas; heavy and indiscriminate shelling of civilian areas; collective punishment; enforced disappearances; widescale and systematic destruction and looting of property; the systematic denial, in some areas, of food and water; and the prevention of medical treatment, including to children - in the period since 15 March 2011.[4]:20–4[7]:4–6[8]:2–4[9]:10–20 Amnesty International reported that medical personnel had also been tortured,[10] while the UN said that medical personnel in state hospitals were sometimes complicit in the killing and torture of patients.[9]:11 The execution and torture of children was also documented by Amnesty International and Human Rights Watch.[1]:30[11]:31–2[12][13] Most of the serious human rights violations documented by the UN have been committed by the Syrian army and security services as part of military or search operations.[7]:4[8]:1 The pattern of the killing, coupled with interviews with defectors, led the UN to conclude a shoot-to-kill policy was operative.[4]:20[9]:10 The UN mentioned several reports of security forces killing injured victims by putting them into refrigerated cells in hospital morgues.[4]:22

Amnesty International decided to enter the country uninvited in spring 2012 and documented "gross violations of human rights on a massive scale" by the Syrian military and shabiha, "many of which amount to crimes against humanity and war crimes". These were committed against the armed opposition, to punish and intimidate civilian individuals and strongholds perceived to be supporting the opposition, and indiscriminately against individuals who had nothing to do with the opposition. In addition to the crimes listed by the UN above, they noted cases of people being burnt alive; destruction of pharmacies and field hospitals (normal hospitals are out of bounds to those wounded by the military); and that the sometimes lethal torture ("broken bones, missing teeth, deep scars and open wounds from electric shocks, and from severe beatings and lashings with electric cables and other implements") was overwhelmingly directed at men and boys.[1]:7–10

http://en.wikipedia.org/wiki/Human_rights_violations_during_the_Syrian_Civil_War