They were controlling 90% of Eastern Ghouta and had Jaysh al-Islam surrounded, didn't they? In fact, quite a few rebels had already left for Idlib by that time, too.
He had Jaysh al-Islam by the scruff of the neck quite a while ago, and they started to negotiate a surrender with Russia before this claimed attack happened.
It's really hard piecing together what's been happening there since the offensive started from all the conflicting sources. From what I gather the opposition narrative is that the regime has been using low-intensity chlorine attacks from the beginning of the offensive, as it has in other cases throughout the conflict. The BBC and other outlets reported these claims in January and February. The claim seems to be that this time the attack hit a particularly enclosed building with little chance of escape and thus the casualty rate is much higher. I know nothing about chemical weapons, but have seen it said that the use of chlorine has not been declared a 'red line' by Western governments.
I've also seen the claim made that the surrender negotiations had stalled and that the remaining JAI forces had dug in for a prolonged siege having let all the fighters from rival groups leave for Idlib and elsewhere.
Obviously I've no idea the actual truth of these claims. The regime narrative on this is obvious and needs no real explanation - indeed they've been warning of a false flag attack along these very lines in recent weeks, which obviously can be interpreted in different ways.
To the claim that Assad wouldn't do this now due to the potential consequences, the opposition notes he hasn't suffered any consequences after previous attacks.
To the suspicion that only civilians, never fighters, are ever killed in these attacks, they claim fighters have been killed in smaller-scale chemical attacks and they wonder why, if the rebels have such access to chemical weapons, they never use them against the regime but only against the people under their control.
I find the reflexive urge to jump in and commit to one particular narrative at this point really strange - you're choosing between the word of a Ba'athist dictator whose most famous war cry is "Assad or we burn the country" and some of the most unsavoury rebel fighters in modern Middle Eastern history (which is saying something).
And why should Jaysh al-Islam give a feck if Assad gasses some of their human shields? It is them holding the people of Ghouta hostage, not Assad.
Well one reason would be that they're from the Eastern Ghouta, it's been their base since they first emerged and it's where they have drawn their recruits from. They're not a foreign jihadi group, they've lasted so long in this area because it's their home. Their impact in other parts of Syria since the war began has been minimal.
everyone's favourite peace loving heroic rebels were controlling it.
At this stage, seven years into the conflict, this is tiresome. Just who exactly is referring to the opposition in these terms anymore? It's years since pretty much anybody had a good word to say about them. Media coverage of the conflict has definitely been skewed against the regime from the start no doubt. And certainly the FSA and associated groups received a good bit of fawning coverage in the first few years. Since the Islamist groups seized control of the opposition the tendency has been to overlook or mention-in-passing the unsavoury aspects of the more 'mainstream' groups like Ahrar al-Sham and JAI, but to the extent that journalists have actually investigated what they stand for, the coverage has been largely negative. I find it far more common these days to encounter Assad cultists who harp on about his supposed 'secularism' and his potential for democratic reform.
On a side note, I find it remarkable just how fully the rhetoric/discourse of the 'War on Terror' has been adopted by everyone seeking to score a political point over Syria and elsewhere. People flinging around the word 'terrorist' as if it actually has any relevance at all in the context of the Syrian Civil War. If you're trying to rationalise just how a regime like Assad's is capable of carrying out attacks such as these, you might look at the dehumanising effect such discourse has.