"Do not forget Aleppo! Do not forget Syria!”  The writer skillfully omitted the comment of the killer that he supported jihad.  

This strongly suggests that the killer is opposed to the Syrian  government and is disappointed that his terrorist cohorts are losing their mercenary war on behalf of US and its Saudi-Gulf State allies.
 
There is no rational that explains the author's contention at all. The resolve to finish off the terrorists is undiminished. Maybe increased.