Demons were simply creatures like that. They trusted only their own kind-in other words, demons trusted only demons. Unless you turned yourself into a demon as well, they would never trust you completely.
To gain complete control over a demon, you had to possess formidable strength. Otherwise, demons would never cooperate without violence.
Besides, unless they had a special purpose or needed temporary cannon fodder, almost no one would summon demons casually. Summoning demons recklessly without sufficient strength was courting death, especially when summoning a Demon Lord.
If you lacked sufficient strength, the instant the Demon Lord was summoned, you would find yourself unable to enslave it. Instead, you would become a pawn in its hands. Demons never kept their word. They believed only in force and obeyed whoever had the stronger fist.
Demonic contracts? Believing in a demon's loyalty would be the death of you.
Kel'Thuzad was the most obvious example. After he summoned Archimonde, Archimonde certainly did not obey his will and become his enforcer. The same had happened to Wilfred Fizzlebang back then. He summoned a Demon Form, Jaraxxus, whom he could not control, only for the demon to tear up their contract the instant he arrived and kill his employer.
Gul'dan exemplified this perfectly. As the world's foremost Warlock, Gul'dan would never casually summon a Demon Lord, especially great Demon Lords like Kil'jaeden and Archimonde. He had never even considered it.
"The avatar of the Sunwell lives nearby. The summoning can begin at any time," Rommath added from beside them.
"Very well done. Once Lord Kil'jaeden arrives, you will receive the rewards you deserve," Doom Lord Kazzak said approvingly.
With that, the two great Demon Lords led their demonic underlings toward Anveena and Kalec's location.
Watching the demons sweep away, Kael'thas's gaze darkened. "Back to Silvermoon