Decision about getting a job while completing MSc Studies Last year I got accepted in a full time informatics MSc. During that period I also had a fulltime job as a software developer in a startup and I was trying to make some publications with my thesis supervisor from my BSc. However all these simultaneous activities had a very negative impact on me, since I had no free time and I slept very little time every day for an extended period of time. Things became worse when a close relative of mine died.
As a result I decided to quit my job and although we managed to publish some papers with my supervisor I decided to end my collaboration with him as well (although it was very fruitful) .
The aforementioned situations also had a great negative impact in my studies and as a result I passed only 2 out of 5 courses in the first semester.
My studies are really important to me as I want to pursue a PhD. Therefore i decided to fully focus on the MSc and improve my grades. So, during last summer I managed to pass almost every course I had failed before, with high grades.
Recently my BSc thesis supervisor called me offering a job in a research program.
This job interests me a lot since:

I would really like to work with him again since we had a great
collaboration...
there is a high chance of publishing again...
I would also earn some money...

However:
I am afraid that this may impact negatively once again:

my studies (despite being close to the completion of the MSc)
my grades (and as a result this could possibly affect getting accepted in a PhD)

I am afraid that if I refuse the offer, he may deny future collaboration.
What is your advice about this ?
Should I accept the job or would that possibly be a bad idea ?
It's clear that you are reluctant to take on full-time employment after your recent experiences, and understandably so. However, this does not mean that you can't take on part-time work to help pay the bills. This is a fairly common phenomenon for master's students, particularly in countries where the master's degree is considered an "undergraduate" degree (such as in Europe). Exactly how much is up to you—you probably know how much time you could spend on external work without compromising your desired work-life balance.

If your bachelor's advisor is understanding and supportive of you, then he would likely be willing to let you work part-time on his project while you finish your studies. If he isn't willing to do so, then that might be a sign that he's not someone you should have as a long-term advisor. (A good research advisor and mentor will want what is best for both of you, not just what is best for the advisor!)