This piece discusses elements from both the anime and the manga so if you are only following the anime or are not up to date on the manga, there will be spoilers so go and make some tea, snack on some Pocky, and leave this article alone until you are all caught up.
Let's look at what we knew about the Titans at the beginning of Shingeki no Kyojin (Attack on Titan). So we know that they just "appeared" over a century ago and started attacking people, forcing humanity behind the walls for protection. The only real interaction from then until the day the Colossal Titan broke through the wall that humans had with Titans was only through the Survey Corps and their missions outside Wall Maria. Upon their return, the Survey Corps would walk through the town battered and broken. Despite the fact humans and Titans have no major interaction for over 100 years, this doesn’t stop them killing every person they come across once the wall is breached.
It is at this point we are only aware of one variant of Titan - the slow, dim-witted humanoid with the toothy grin that makes them appear almost indifferent to the pain and suffering they inflict. But as you continue on through the story and learn more about them, you become aware of the fact that they are not as mindless as previously indicated with their entry through Wall Maria. We learn that they instinctively hunt out humans, acting on some sort of natural compulsion to seek them out. But after Eren is swallowed (by what looks like a giant Santa Claus), we see that the humans who were eaten are not digested as expected. In fact, according to research carried out by the Survey Corps, it appears they don’t actually need to eat. They even ignore all other wildlife in pursuit of human prey. So rather than a need to feed, there appears to be no real biological reason to consume humans. There is no apparent cause for the destruction they bring to men, women, and children. This form of unreasonable genocide leaves the reader with unanswered questions. Why kill humans? Why do they ignore all other living creatures? What purpose does this serve?
Add to this the general size of a Titan, their ability to regenerate after injury, and the one and only weak spot being located on the back of the neck making them a difficult target to stop, they are the ultimate killing machine. Witnessing the consumption of characters we get to know along the way doesn’t help our feelings towards them. The moment we see Eren’s mother killed is designed to evoke compassion and sadness and when we see the Titans work their way through their human buffet, we know little about the people being butchered. But when we see Thomas hanging out of the mouth of one before he is bitten in half with a look of horror on his face and that on the faces of his friends, for the first time we as an audience are able to identify with a victim. As we see more and more popular characters die, we see the effect it has on those left behind and the emotional strain that the horror of witnessing the slaughter around them has had on each psyche.
Of course this is all concerning your average run-of-the-mill Titan. We have not even begun to cover the Titan Shifters. Eren’s transformation is a twist enough but finding out the new and stronger Titans who set about the destruction of the wall – the Colossal Titan and the Armoured Titan are able to walk among everyone in human form just like Eren and Annie is the ultimate shock. The very Titans responsible for breaking down Wall Maria, Bertolt and Reiner even manage to join the Survey Corps, effectively infiltrating the enemy to show us a new side of the Titans we didn’t see before. We as the audience were used to seeing these monsters as mindless killers working on an unknown biological instinct but now we see intelligence making them even more deadly. This fear in increased with the introduction of the Beast Titan – a werewolf/ape type Titan capable of communication. Not only are humans outclassed in size, strength, and ability, they are now faced with an enemy possessing the one tiny thing they had on their side: the ability to think. Battles can be won on tactics and now that one thing on humanities’ side can be easily turned against them.
But the scariest thing about Titans is not the killing or their appearance. It is that fact that humans can become them. The Beast Titan has the ability to transform people into Titans. We even see this with Connie’s mother whom while appearing to be a run of the mill savage killer, recognizes her own flesh and blood. Imagine the horror of having a loved one look back at you through different eyes. The eyes of something you perceive as evil. We see this with Eren too and his father who turned his own son into a Titan and allowed himself to be devoured. Is this a really bizarre way of showing love that could have been avoided by just giving your kid a stuffed animal or a passing on a genetic baton from father to son? Eren’s father must have had his reasons. After all, these powers were originally intended for the Royal Family - The people living inside of the innermost wall and as far as many of the citizens are concerned, have caused almost as much suffering to the people as the Titans. Maybe that is the real reason why Titans are so fearsome. Not because they kill or their invulnerability but how closely they resemble humanity in all its malice.