can a bacterium cell become a cancer?
You ask two questions:


If a bacteria can get the same characteristics as tumor cells. 
If such "bacterial tumor cells" actually can form a "tumor" within the
human body.


I assume that your question is not if bacteria can change in such a way that the can be confused with eukaryotic cells. This they cannot do. Bacteria cannot form organelles. Because they have no organelles they are solely dependent on their membrane for energy production. Because the surface area to volume ratio decreases with cell size there is a physical possible maximum size for bacteria. Here is some more in depth info on the subject.

So what characteristics can bacteria then have that tumor cells also have?
Well, according to wikipedia, a tumor is a mass of abnormal and excessively grown tissue (neoplasm) which may or may not be malignant. According to this definition bacteria could be tumor-like if the could form tissue and if this tissue could then grow abnormally or excessive. 

And guess what? If you define tissue as "an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function." like Wikipedia does. Then you could argue that the biofilms some bacteria form are quite tissue-like since biofilms also contain an extracellular matrix, the cells within have the same origin and (in some cases) the bacteria can have differentiated functions.

However, the fact that bacteria can form biofilms does not make them tumor like. For that comparison to work mutated bacteria should be able to outgrow the rest of the biofilm. And while mutation that cause noncooperative/cheating/abnormally growing bacteria in biofilm are known, this kind of mutations cause the biofilm as a whole to grow slower. (source) In this sense comparing a biofilm to a colony of ants may be more appropriate than comparing it to tissue.

So to answer your questions:


Bacteria can form tissue-like formations, but when members of this formation do not cooperate anymore this typically leads to less growth, instead of increased growth. 
Biofilms do form within humans. For example the bacterium Pseudomonas aeruginosa is assosiated with cystic fibrosis. And while cheaters exist among these bacteria, biofilms with high ratios of cheaters have a lower fitness. see this paper Therefor you would expect most biofilm within humans to be more tissue-like and less tumor-like.