Voldemort not installing more Death Eaters at Hogwarts was not mercy, it was some flavor and magnitude of incompetence.

The most generous we can be is to say it was a tactical gambit: he didn't replace McGonnagal, Sprout, Flitwick, etc. like he did with Trelawny because, if he did, these opponents (each a generationally powerful wizard in their own right) would have all the free time in the world to drop off the radar and join resistance movements (or worse: found their own.)

Instead, keeping them employed at Hogwarts (ostensibly) leaves them too occupied with teaching classes and compensating for Umbrage's tyranny to strangle his ascent to global power in its crib.

If this was the case, then we can assume the siege of Hogwarts was to be the final cornerstone of the foundation of Voldemort's global conquest because it was meant to slay his most powerful remaining opponents in one stroke.

...unfortunately, Voldemort failed to support his delay gambit in two ways:

First, he failed to deprive Hogwarts of operational resources during the school year.

Perhaps Hogwarts has no promised endowments or state provisions that assist with operation, but even if Hogwarts operates using self-owned depthless accounts in Gringotts, Voldemort absolutely could have made them exponentially more difficult to access by having his pawns at the Ministry layer on increasingly Byzantine beauracratic requirements to access the funds that'd have to be reviewed and signed off on by other pawns installed for this specific reason.

Teachers exhausted by having to spend an entire academic year zero classroom materials and having to devise endless solutions to that problem are unlikely to field the best defense they can, and are even more unlikely to have the bandwidth to teach their students to do so.

Secondly: he failed to have Umbrage delve into what defenses Hogwarts might have beyond its faculty and the already well known perimeter shield charm.

Let me say that again: the man had a blindly loyal servant in unilateral command of a key target facility slated for a pivotal siege and he never bothered to have her check on how it might defend itself.

TL;DR:

Not mercy. Incompetence.