There are unfortunately those who exist at the lowest, basest possible consciousness level. These people are what traditional religionists would call evil and psychologists/psychiatrists would call sociopaths or psychopaths. Such people do not follow nor believe in the concepts of right and wrong. They are also soulless and consciousness. It is not unusual for such people to betray other people for their own means. In fact, they consider such behavior to be normative morality.
These people betray others the way normal people care for others. They view people as mere fodder to accomplish their particular ends. They want what they want regardless if others are hurt or damaged in the process. To them, people are just mere pawns that they use at will. Oftentimes, these people will betray others as a part of gamesmanship. They have no loyalty nor concern for others but only for themselves.
However, in extreme circumstances, even good, moral, and upstanding people betray others just to survive in such extreme circumstances. For example in concentration camps, a prisoner can betray another prisoner just to stay alive yet another day. Or that prisoner can betray other prisoners in order to obtain a coveted position which is easier for them.
In other situations, people betray each other to obtain success or to curry favor from a more authoritative person. For example, a sibling may betray another sibling to incur parental favor in terms of preferential treatment and/or other family perks. This sibling may want to be considered by parents "the golden sibling" in comparison to the other sibling/siblings in the family.
People betray each other in order to get revenge. They want the betrayed person to get his/her just desserts or simply feel the hurt and anguish the former felt by being wrongly or unjustly treated. Lastly, envy leads some people to betray other people. They figure that since their circumstances are less than ideal, seeing that the other person have more ideal circumstances, the former intends to bring the latter down to their level. The idea is that if their life is less than positive, then others will be also be less than positive.