After her fateful conversation with L'Rell, who told Cornwell that the only way the Klingons would be stopped is by killing all or most of them, Cornwell and Starfleet's decision was completely understandable from a tactical standpoint, so I don't think Cornwell was made to look like the bad guy at all.
...
And just like Burnham, who made the rational decision to seek counsel with someone with superior knowledge of the enemy in the form of Sarek, Cornwell sought out L'Rell. who told her that the Klingons wouldn't stop until they were all, or mostly all, dead.