There are a few ways Summoning a spirit can become dangerous:
1. Summoning the wrong spirit. The Spirit World is much like our own; it has good and bad. Be careful and always be aware of what you are Summoning. Always summon the right spirit for the right goal, and it will happily help you. Not because you are above it, but because there is common interest.
2. Believing that you are above the spirit. This is a very dangerous mentality, and it can cause the spirit to either refuse to serve you, to trick you, or even harm you directly. NEVER consider yourself above a spirit. You are its equal. Nothing more, nothing less.
3. Believing that you are beneath the spirit. Again, a very dangerous mentality. This belief can cause the spirit to take advantage of you and either trick you or even possess and/or attack you. Once again, you are the spirit's equal, working together towards a goal. No more, no less.
Also, there are MANY more types of beings that can be summoned other than Demons. Some Practitioners can even summon Astral beings in addition to Spiritual/Etheric beings.