Highschool DxD S2 - 5
Why did an angel summon the demonic guards of Hell? Wouldn't they be loyal to the demons?
He is a fallen angel.
And besides in the dxd universe , hell is another realm ( demon realm , heaven , hell , human world are exemple of realms ) all separated by the great void. ( i won't get into detail what the greta void is but it's important ) All realm are linked together by several pathways.
Now the rule to not forget.
All beings from a realm can be summonned in another realm given that you met the proper conditions ( that are more insane the stronger the thing you want to summon and the place you want it summonned )
Aside from the fact that hades ( master of hell in dxd universe ) is a giant *sshole and allow it , there is the fact that this guy is so high spec that he can summon him because he fullfilled all the right conditions beforehand.
I can answer other questions if you wish.