If it's an assignment for school and they're telling you what to argue, then you argue that. You don't have to believe something in order to argue for it. Indeed, a decent writer will be able to argue any position, whatever the topic. Examining arguments for a position you don't agree with might help clarifying your own position. So think of writing this paper as an opportunity to flesh out your own ideas; write what you're supposed to write but use it to make up your own mind.

I do believe it would have been better if your teacher had just asked for your opinion on the subject, for or against, as long as it's supported by proper arguments - but that might be too difficult to grade. Another exercise might have been to ask you to write two papers, arguing for and against the topic - but that might be too much work for you (and it would double your teacher's workload). In any case, you're stuck having to argue for a position you don't believe in. Tough luck.

So what you do you do? Think of it as an exercise in writing a piece of propaganda. You pick whatever positive things you can find (you've already identified some) and emphasize those. Mention some negative things, but downplay them (don't say anything stupid or offensive while you do, stay classy). I'm not sure at what level you are and how much work is expected of you for the paper, but perhaps have a look at some sources from the late 19th, early 20th century. That's generally the prime period in which the colonizers saw themselves as bringing benefits to "backwards" people. Take this soap ad for example, it mentions "teaching the virtues of cleanliness" and "brightening the dark corners of the earth as civilization advances". It references the famous poem The White Man's Burden, a poem which includes the lines:

Take up the White Man's burden 
And reap his old reward: 
The blame of those ye better, 
The hate of those ye guard

I hope my remarks help. Good luck with your assignment!

I would personally imagine that this is more of a "meta" attempt at teaching, not by trying to convince you of the merits of European colonialism, but instead someone somewhere did indeed think it was a good thing. Right now you can't rationalize how people were able to see it as a good thing, but clearly someone, somewhere did.

The teacher is trying to give you an opportunity to explore this possibility, even possibly lying to yourself or giving yourself half truths to justify a position, that way you are receiving a proper understanding of what someone has to do to convince themselves that going to Africa and setting up a Belgian Congo is a good thing.

So in this case, cast yourself as a semi affluent European at the time of colonialism and try on arguments that seem palatable that would let you believe that Africa should be plundered.