Jikan Teishi Yuusha - Yomei 3-ka no Settei ja Sekai wo Sukuu ni wa Mijika Sugiru - Ch. 46 - Brutism and Stop

[Telumire]

assuming that Darklings are bad and it's ok to experiment on them, how did she know this? Why not experiment on human, or elves, or dark elves? Unless she already experimented on them all to reach that conclusion, which again, will make her the most evil person in the series.

Pure speculation but maybe they tried to invade land in the past ? In time of war experimenting on living being can be "forgiven" if you are on the winner side, so my guess is that she took advantage of that and experimented on darkling due to their immortality (elves lives are long but they are not immortal in this world). But yeah regardless of if they actually are a murderous specie, they still are capable of feeling pain so experimenting on them is immoral, for sure.