One bad apple can ruin an entire basket of apples and this applies to a bad salesperson in a team of good ones; it is possible for the whole team to come to their knees just because of this one person. When doing sales assessments, it is important to remember that, a bad salesperson can take different forms and shapes.
Some people might look at bad salespeople as those who are always poor when doing their job and end up missing their quota, but it is possible that, a high-end salesperson that exceeds their quota to still have a bad influence on the entire team. This could be through being hubristic.
A top performer in sales who finds their name at the top might develop pride. The extra attention and visibility they receive could lead to them being too arrogant. In the process, create a workplace that is emotionally charged. If not controlled, it will precipitate to petty resentments and jealousy among the rest of the team. Being as competitive as they are, they might refuse to share the secret to their success. This type of selfishness and pride at the expense of the entire sales team might ruin everything.
It is very easy for bad habits to spread easily. It is a well-known factor that, most top-performing salespersons are lone people, working with their own files, outside the system. The young and upcoming sales representatives might see this as a sign of success and want to copy their seniors. But truth be told, what works for someone else, might not work for you and that is why a good team should be able to teach and live to a process which works for the entire group. When you have in your group a sales representative who encourages habits that are bad, then it will be the detriment of the entire team, then you will need to take care of them before their bad influence gets rooted in the entire team.
The sooner you do this, the better for the entire team and everyone will be happy.