The church just want all the soldiers to use it so they all die off and the church army can easily conquer everything (they have this plan to unite all humanity under their banner to basically goes Warhammer Imperials on the Xenos (Devils).
Garcia used the pills on the southern people and some of her soldiers, and when the church stop supplying her with pills (either because she realized their goal, or this made her realize their goal), she sacrificed the southerners first, but her soldiers eventually die anyway making her turn hostile against the church.
(they eventually 'kill' her)
Timothy on the other hand, is too dumb to realize the church's plot but 'smart' enough to stop using it on soldiers/knights, and just give the pills to forcefully conscripted peasants and use them as basically suicide chargers.
Needless to say, Roland was PISSED at Timothy wasting so much human lives like that and this became part of the trigger for Roland to seriously take down Timothy.
Btw Roland's youngest sister is a witch whose ability is a passive stat buff (increases INT), and eventually she form alliance with Roland and is one of the few people who can converse with Anna on the more advanced topics like physics.