Consideration:
Before you pick up your wand with the intention of casting dark magic, you must consider the effects.
You may wonder what these effects are. Isn't dark magic just “Oh I did something bad, but nothing will happen”? Well, sorry to tell you, but that is not the case. There is an effect to performing dark magic, and this needs actual thought. You cannot go flicking around your wand without thinking of what is to come of that. There are many things you must consider, such as the law and more.

The Law:
Yes, some forms of magic are banned from wizarding society, and you should always be informed about any banned Magic. One of these banned magical arts are the Unforgivable Curses. As you know from your first year, there are three unforgivable curses- Imperio, Avada Kedavra, and Crucio are their incantations. Their names are the Imperius curse, The Killing curse and the Cruciatus curse. You need to consider the effect of these curses in cases of the law.

Performing these spells can lead to being jailed, and put in Azkaban- A prison with dementors to prevent your escape. Another way to get yourself in trouble is using Horcruxes. Horcruxes are pieces of your soul in another item/being. You will learn about Horcruxes in later years, but creating a Horcrux is banned, and can lead to an ending in Azkaban. This is the reason of why you need to consider the law before using dark magic. Killing others using any of these methods, whether they're Muggles or not, will lead to you being put in Azkaban, a place most people don't survive. If the magic is performed on a Muggle, charges will be far worse as Ministry officials have to go through and put memory charms on the Muggles involved.

Your Soul:
As we know from your first year, Casting the three unforgivable curses can result in there being a print on your soul. This print cannot be removed, once you've performed the curse it will already have an effect on your soul, causing it to turn evil or “rotten” as some would say. Back again to Horcruxes. Horcruxes split apart your soul and it CANNOT be put back together. That is an effect on your soul that cannot be fixed. Unless you want to be torn and scarred for life, I wouldn't cast these spells. It may be that at the time of doing it, it may seem fun. Trust me though, later in life that may not be the case, you could very well regret it. Damaging your soul is damaging you forever, some don't take this into consideration and end up how Tom Marvalo Riddle did, or as you may know him, Voldemort. You have/will learn about Voldemort in this year- your 2nd year in Hogwarts. Essentially what you need to know about him in this case is that he didn't really consider what would happen to his soul before he performed banned magic, and this took effect on the person he was, and turned into the Dark Lord we all feared.

Others:
Of course, dark magic is used on others, as you wouldn't cast the Killing Curse on yourself. You can effect others with the Unforgivable Curses, depending on which curse you use. Any Jinx, Hex or Curse would have an effect on another being. From simply making your victim uncontrollably dance, or even possibly death, it is still an effect on the victim. This may be fun and games, but if the spell is casted on a Muggle, this could lead to a life sentence in Azkaban. Even if casted on a witch or wizard, you could still find yourself in trouble. I highly suggest you don't do this without permission.

Human Life:
Dark Magic isn't good for animals, mostly Human life. For very obvious reasons, using unforgivable curses on Humans is a very bad idea, as it has clear consequences. I don't think anyone can say that death is good for a Human's health. Going back to the subject of Horcruxes, Horcruxes aren't good for Humans either. Let's take Harry Potter as an example of this. Potter was the “boy who lived.” He survived the killing curse, casted upon him by Voldemort. Voldemort, after he failed to kill Potter, lost all his power, and while he lost his power (body) he made Potter into his final Horcrux. This made it to where Potter almost got into Slytherin, Voldemort’s house, and whenever Voldemort is feeling Happiness or Anger, Potter knows. This also had an effect on Potter’s wand. Potter got a Holly with Phoenix wand, and the Phoenix feather in his wand is from the same Phoenix in Voldemort’s wand. It is thought of to be the Phoenix Fawkes, that belonged to Albus Dumbledore. A Horcrux also has a terrible effect on nearby people, creating distrust, anger, and even betrayal.

Summary:
This is why we MUST consider the effects of Dark Magic before you perform Dark Arts. A person who did not consider the effects, was Voldemort as we know from before. He casted Dark Magic freely, and not caring. This is why he wasn't very Human-like, his soul had “rotted” and was made into Horcruxes, leaving little to no humanity left inside him. Everything you consider seems to lead back to another thing, as when considering others leads aback to the law.

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Quiz and homework suggestions. *You don't have to do this in particular*

What happens when you don't consider the effects upon your soul?
What happens when you don't consider the effects upon others?
What happens when you don't consider the effects involving the law?

Homework:
What do you have to consider before performing dark magic?
Why do you have to consider before performing dark magic?
(Doing both in one is allowed)