So your answer is yes, you quantify barbarity and reprehensibility by their reach and the amount of people they kill?
From the the Iron Guard and Lancieri of old to the Klan, Army of God, NLFT, NSCN, Hmari militants, the MNCA, Maronites, LRA, LoC, CC, CSA, the Hutaree, PP, ARA, and hundreds more (to say nothing of the individual terrorists who take it upon themselves to murder abortion doctors and bomb abortion clinics), there are PLENTY of Christian terrorists who do what they do in the name of a twisted view of their religion.
Are you really arguing that ANY of those, regardless of whether they kill just one person or hundreds; or whether they're local or world-wide, are any different?
If you are, I think we've found the problem.
Yes, I'm really arguing quantity matters lol.
You have two problems: the most obvious one is it's important to you for some reason that Christianity be on equal footing with Islam in the terrorism department. But it just isn't so, and the numbers tell the tale.
Secondly, in attempting to do this you are giving inadvertent credence to the proposition that groups such as ISIS and al Qaeda aren't really Islamic.
There is such a thing as perverting religion for nasty purposes. The LRA is shining example of it. They call themselves Christian but their 'doctrine' is an incoherent hodgepodge. Clearly, they are a tribal group that took on a Christian sounding name and they may go on about the Ten Commandments but you won't find a single theologian that would either claim them as fellow Christians or recognize their 'theology' as being Christian or rooted in Christianity.
This isn't to say there are no Christian terrorists or terrorism. Since abortion bombers are acting on Christian motives they can be fairly categorized as Christian terrorists. The KKK are/were better organized [the Klan is largely a relic]; but even in their case, rank racism is clearly more important to them than Christianity. Basically, they cherry pick the Bible, when they even bother to mention it lol, in support of white supremacy. So they rank as perverters of religion.
In contrast, radical Islam is a coherent theology rooted in the Koran and the later writings of Muslim jurists. Furthermore, there is plenty of historical precendent for ISIS that is fortunately lacking in the case of the LRA or abortion bombers. Or the Klan. Which neatly explains why Christian terrorism tends to be a localized phenomenon.
The bigger reason for that is part of radical Islam's coherent theology is the spread of Islam via jihad. Jihad can take many forms, it needn't be violent.
And when it's violent, radical Muslim groups can easily cite Mohammed to justify their actions in ways so-called 'Christian' terrorists can't. The latter are easily and roundly condemned, and refuted, by the global Christian community. Which again, explains why it's always localized and not global.
Which makes it a qualitatively different phenomenon than Radical Islam.