What did you expect with Aleppo?
Syria's hereditary socialist/nationalist (Ba'athist) dictatorship has flagrantly used chemical weapons against its own people and dropped barrel bombs on them, for daring to oppose 46 years of repressive family rule. Nobel Peace Prize winner, Barack Obama, said the use of chemical weapons would be a "red line", then did nothing besides let Assad (and his father) 's ally Russia "supervise the destruction" of the weapons.
Obama, leader of the world's only superpower, then did nothing. His excuse was that the UK House of Commons had voted against military action against the Assad regime (which it did, as now former leader Ed Miliband wanted to prove how much the "anti-Blair" he was and burnish his leftwing credentials). Non-intervention, the preferred policy of rightwing isolationists and leftwing "pacifists" is the new norm, except for Russia.
Of course it isn't pure non-intervention. The West has been funding and arming some of the rebel groups in Syria, including those with Islamist leanings. They aren't ISIS (despite some claims), but there are no angels in Syria. No one is fighting for Syria to be a secular liberal democracy that respects individual rights and political plurality.
The surrender of Aleppo to the Assad dictatorship was the inevitable outcome of Russian intervention in favour of its long standing ally and the flagrant ongoing violation of international law by the Assad regime in using chemical weapons and barrel bombing civilian areas.
Chemical weapons and indiscriminate bombing by the Assad regime has worked. China, Russia, Iran, all of which execute political opponents, don't care. The part of the international community that should care (the "West") has shrugged, said lots, but Obama handed over responsibility to Russia. This was like handing over responsibility for addressing North Korea's human rights atrocities to China.
The experience of Iraq - successfully overthrowing tyranny, followed by utter failure in replacing it and achieving control of the country, rightfully gave cause for caution.
However, the result of that vacuum has been to give Russia an opportunity, to be the new power in the Middle East. There were opportunities to contain Assad's use of chemical weapons and air power over civilians, with no fly zones, but doing anymore would have been much more difficult.
Now those on the left are complaining that "we" sat by and did nothing, yet that is exactly what they campaigned for. Dictators will murder opponents, will slaughter civilians and unless you are willing to put our own taxpayers' money and military force to intercede, it will continue. Obama in 2011 said Assad either had to lead a transition to democracy or get out the way, but he did neither - he fought on, gained support of his strong ally - Russia - which already knew the West was going to do nothing.
The isolationist right of course also believed in leaving Syria alone, a few because they accepted Assad's propaganda that all his opponents are "terrorists" and all his opponents are "Al Qaeda and ISIS" whereas he is moderate and reasonable. A few because they see Putin as a "friend". However, mostly because they have no interest in what happens to people in foreign lands, as they are far away places of which they know little. Syrians wanting freedom from tyranny should do it themselves, and not expect foreign government support (even if it means foreign governments actively support the tyranny). At least that position has a consistency - governments should only defend the rights of those within their boundaries, even if other governments engage in mass slaughter that sends hundreds of thousands fleeing to other governments.
So Aleppo is awful. Yet, it is the end result of the policies of both leftwing so-called "peace" supporters and rightwing isolationists. The biggest threat to the lives of individuals are tyrannies, and the only way to redress that is to arm opponents or to take them on yourselves. The Western appetite for this is slim indeed.