Tom Riddle is here-Hogwarts Slytherin Side

1st, 2nd, 3rd

Let us first begin with the biggest controversy, and the reason that the current generation believes all Slytherins are evil. That a majority of Voldermort's followers were Slytherin. Now, this is not the sole reason Slytherins are seen as vile creatures, if you note, the hatred between Slytherin and the other houses has been a long time in coming. However, considering this is one of the first things that comes to mind nowadays when a person thinks of Slytherin, and just how bad that they must be. So let us think about this. 
Voldermort, well known as the darkest, most dangerous, and evil wizard to date. He was placed in Slytherin, as were all of his predecessors, and it is now well known that Voldermort is a direct descendant of Salazar Slytherin himself. He was a half-blood, but was commonly seen as a muggleborn because of his lack of knowledge of which parents were magical, or even if they were magical (though this was certainly something that he assumed, that he had at least one magical parent). Yet, he was not treated as badly as most muggleborn's. Why? Because people were intimidated by him. They were intimidated by his power, by his control, by his actions and willing ambition. He was accepted because he acted as a Slytherin should. He was cautious, he was careful, and while many knew of his doings around the castle, none could prove them. He was intelligent, he managed to swindle all into believing he was harmless, despite the reality of his dark aura. Yet, he had power. Not just magically, but he was influential. People wanted to listen to him. 
It was this that caused him to be able to begin to gather so many followers. Think on it? A slytherin would know how a slytherin mind would think. Swaying Slytherins to his side would prove far easier than even another Hufflepuff. Certainly much more fruitful than it would be to gain followers among the Gryffindors or even Ravenclaws. Voldermort was no idiot. Most Slytherin families have some connections, others, plenty of connections. Given their bad treatment at school, it was not exactly difficult to persuade most Slytherins. Promises of power-so that nobody could hurt you again-promises of revenge against those who dared bully you or cause you pain and discomfort. The power to allow none of this to ever happen again? The power to be able to defend yourself, and protect yourself. These are promises that Voldermort gave to entice the Slytherins. We are not all that complicated, at least to a mind of a Slytherin. Another Slytherin knows where to poke, knows what to offer to gain the affections and loyalty of Slytherins. It is not all that complex. And at this time, Slytherins were treated especially awful. From the hatred of Grindelwald and Slytherins being blamed for this, many were seeking some form of relief from this pain. They wanted to not have to worry about themselves, or their lives. Voldermort knew that, and therefore he could use it. Slytherins would prove the most useful, hence why he focused most of his attentions on them. Though I think it obvious that he could have chosen any of the houses, none of the other houses had quite as much influence or power as the Slytherin house did, and does.
After he managed to secure his following, it became easy to manipulate them into following him and never betraying him. We all are of the understanding that Voldermort was dangerous. He cared little for others. This, compiled on the fact that he was an accomplished legitimens caused others to fear betraying him in any way. He managed to slowly turn his followers from loyal out of a common desire, to loyal out of fear. Though many followed him most willingly, others dared not betray him out of fear, and even his most loyal followers held a healthy degree of fear. Voldermort was not shy to torturing even those whom he would call friends. It was this that showed his true cruel nature. And while Slytherin is not shy to cruel people, that too, is from circumstance. Especially among the second generation of followers. 
Many second generation followers were raised in the presence of Voldermort. They were desensitized to the harm and cruelty that the first generation had not been quite used to. It became a common occurance. It was something normal. They were practically raised under Voldermort's thumb, and this caused him to have even greater loyalty-in spite of fear-from the second generation, than the first. He needed less force with the second generation, as they were practically molded to what he desired out of his followers. Respect, Fear, Admiration, Loyalty, Worship. Because of this, even if he tortured the second generation of followers, they still admired him, they twisted the thought wondering what they could do to better themselves. He was like a god to them. He could do no wrong. Any wrong was on their end. Those who gained the trust and belief of Voldermort were instantly the ones to strive to be, but by default, these were also the ones who were the most cruel, the ones Voldermort had the biggest hand in influencing. Raised as his personal torturers, the glee they found in others pain. He raised psychopaths. But, to this end, the third generation were different.
Voldermort was already dead when the third generation were growing up. Therefore he was unable to have such a big impact on the third generation lives as he had had on the first and second generation. However, the way the second generation followers were raise...this negatively impacted their children. Because of a lack of pain to cause others, and the joy they received from watching others in pain, they turned inward. Onto their own children, and their servants. So their servants and children were then treated as the muggles that they tortured once were. Punishments unimaginably cruel placed upon their own blood, and the sadistic pleasure they derived from it. There was no escaping the cycle. 
And yet, somehow, out of the three generations, the third generation shows by far the most Slytherin traits. Why is that? I shall explain that as well.