The title should explain it all. Many (perhaps most) Christians view Tarot cards as evil, or at least something to be avoided. Why?
Tarot cards are widely used for fortune telling and other occult practices, which most Christians consider to be sinful. While the cards may not be themselves considered bad, or possessing them inherently sinful, it is likely to be thought of as an indication that you are involved with occult practices, or are laying yourself open to temptation in that regard.
Having a good reason for possessing Tarot cards, not involved with occult practices, is going to be looked on differently. The Tarot deck is in fact just a standard card deck from the 15th century, and only came to be associated with occultism in the 18C. There are a number of historical games (some of which I play) that use the deck, and have no occult connections at all.
For the specific case of the Roman Catholic teaching, the principle objection to the Tarot (as a fortune telling means) is that it is either superstition or divination. Both are prohibited / sinful as they contradict or interfere with following the First Commandment. The current Catechism has a discourse on the First Commandment that includes commentary on those two practices.
2111 Superstition is the deviation of religious feeling and of the practices this feeling imposes. It can even affect the worship we offer the true God, e.g., when one attributes an importance in some way magical to certain practices otherwise lawful or necessary. To attribute the efficacy of prayers or of sacramental signs to their mere external performance, apart from the interior dispositions that they demand, is to fall into superstition.
A few paragraphs later, divination is covered as well.
Divination and magic
2115 God can reveal the future to his prophets or to other saints. Still, a sound Christian attitude consists in putting oneself confidently into the hands of Providence for whatever concerns the future, and giving up all unhealthy curiosity about it. Improvidence, however, can constitute a lack of responsibility.
2116 All forms of divination are to be rejected: recourse to Satan or demons, conjuring up the dead or other practices falsely supposed to "unveil" the future. Consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers. They contradict the honor, respect, and loving fear that we owe to God alone.
A prohibition against divination magic can be found in the Old Testament.
Let there not be found among you anyone who immolates his son or daughter in the fire, nor a fortune-teller, soothsayer, charmer, diviner, or caster of spells, nor one who consults ghosts and spirits or seeks oracles from the dead. Anyone who does such things is an abomination to the LORD...
Many (perhaps most) Christians view Tarot cards as evil, or at least something to be avoided. Why?
The practice of divination is abhorrent to God.
Deuteronomy 18:10 There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch,
Engaging in occult practices brings one under greater demonic influence.
Isaiah 8:19 And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead?
The discerning Christian desires to avoid Satanic traps.
2 Corinthians 2:11 Lest Satan should get an advantage of us: for we are not ignorant of his devices.