You have three questions to answer and 100 records per month to analyze.
Based on this size, I'd recommend doing analysis in a simple SQL database or a spreadsheet to start off with. The first two questions are fairly easy to figure out. The third is a little more difficult.
I'd definitely add a column for month and group all of that data together into a spreadsheet or database table given the questions you want to answer.
question 1. Users who are consistently showing up in this list
In excel, this answer should help you out: https://superuser.com/questions/442653/ms-excel-how-count-occurence-of-item-in-a-list
For a SQL database: https://stackoverflow.com/questions/2516546/select-count-duplicates
question 2. Users who are consistently showing up in this list with high risk score
This is just adding a little complexity to the above. For SQL, you would further qualify your query based on a minimum risk score value.
In excel, a straight pivot isn't going to work, you'll have to copy the unique values in one column to another, then drag a CountIf function adjacent to each unique value, qualifying the countif function with a minimum risk score.
question 3. Users who have/reaching the high risk level very fast.
A fast rise in risk level could be defined as the difference between two months being larger than a given value.
For each user record you want to know the previous month's threat value, or assume zero as the previous threat value.
If that difference is greater than your risk threshold, you want to include it in your report. If not, they can be filtered from the list.
If I had to do this month after month, I would spend the two hours it might take to automate a report after the first couple of months. I'd throw all the data in a SQL database and write a quick script in perl or java to iterate through the 100 records, do the calculation, and output the users who crossed the threshold.
If I needed it to look pretty, I'd use a reporting tool. I'm not particularly partial to any of them.
If I needed to trend threshold values over time, I'd output the results for all people into a second table add records to that table each month.
If I just needed to do it once or twice, figuring out how to do it in excel by adding a new column using VLookUp and some basic math and a filter would probably be the fastest and easiest way to get it done. I tend to avoid using excel for things I'll need to use with consistency because there are limits that you run into when your data gets sizeable.