To newspapers or TVs stations, when there were no cell phones , thanks for the heads upNot everytime but very often the IRA would call ahead with a warning before the bomb went off.
Terrorists from every religion have been known to target civilians and "seemingly to reach the highest body count possible and to make the biggest public display of horror" has always been the point of it. That includes IRA and PLO (they are Muslims btw). IRA used to blow up pubs and PLO member organizations were best known from a school bus massacre (killed 9 kids) and the Munich massacre that targeted Olympic athletes.
Pretty much every terrorist organization have sacred religious texts that give them justification for their actions. It's always some variation of end justifying the means. Usually they are the only ones that draw that conclusion for what ever material we are talking about (Quran, Bible, Torah, Das Kapital, Mein Kampf ect.).
ISIS and friends aren't some special snow flakes that are doing unprecedented things. They are very much derived from the past. They are much wealthier than previous organizations and they spend a lot more resources on spreading propaganda compared to their predecessors. But those are pretty much the only differences.
Africa and middle-east were ahead in the slave game simply because that's where the earliest civilizations were formed. Ancient Greece and Rome got their slavery on as soon as they were advanced enough. How brutal any of it was is something no one knows. Something any decent historian can tell you. No idea what slavery has to do with anything quite honestly. It's endearing that you deem it important to mention that Roots didn't invent slavery, but that has more to do with your "education system" than anything that's relevant to this thread.