Buying weapons doesn't necessarily mean that Bangladesh wants strengthen borders nor to attack her neighboring countries.
Don't forget guys that Bangladesh earns big junk of remittance from UN peacekeeping operations. Bangladesh sends the highest number of troops in those missions and that number is increasing. So, obviously Bangladesh military needs more weapons and equipment if they want to contribute like that. Now, whether Bangladesh should have gone to China or Russia is a different issue. From a historical point of view, China opposed Bangladesh's liberation war and Russia supported us. So, this government felt comfortable dealing with Russia than with China. Moreover, we all know about China products