Hi doma,
No one wants the Sunni extremist terrorist groups to be successful except the Sunni extremists.
They are opportunists.
Unfortunately, their opportunism is poisonous to the free Syrian cause.
To a great extent, this is no longer a fight for freedom against repression. Instead it has become a proxy war between Shi'ite Moslems and Sunni Moslems.
But it isn't even that neat. Civil wars are a mess and you only find out which faction won after the war is ended. eg in Egypt, Islamic traditionalism and secular democracy defeated dictatorship, and then Islamic traditionalism defeated secular democracy.